The letter to the Ephesian church is regarded as one of the masterworks of the NT, but it is also one of those books that the critics like to say wasn't authored by Paul. Are they right? In this article we'll be looking at the arguments offered on both sides.
Our study begins with a primary thematic consideration: What is the purpose of Ephesians? Certain clues tell us of the need to adjust our expectations:
- Ephesians contains more hymns and liturgical material than any other letter. Barth reasons therefore that certain aspects of Ephesians -- it's unusual diction, and more (see below) -- are attributable to its considerable use of hymnal and liturgical material from within the church. He finds such material scattered throughout Ephesians.
- Ephesians is not always found attributed to Ephesus. The manuscripts mostly attribute the letter to Ephesus. But some mss. have no attribution at all, and Marcion possessed Ephesians under credit to the Laodecians. [Mitten, 3] In addition, the closing (6:23-4, "Peace be to the brethren, and love with faith, from God the Father and the Lord Jesus Christ. Grace be with all them that love our Lord Jesus Christ in sincerity. Amen.") appears to be general. Finally, some passages (1:15, 3:2-3, 4:21) suggest that Paul does not know the readers personally, which contradicts, on the surface, the testimony of Acts that Paul was heavily involved in the Ephesian church.
- Ephesians is clearly intended for a Gentile readership.
Therefore, it is our thesis that Ephesians:
- Was a general letter, or an encyclical, to be delivered to a broad range of primarily Gentile churches, just like 1 Peter and James
- Used churchwide liturgical and hymnic traditions to verify the correlation of Paul's thoughts with those of the rest of the church
- Survived mostly as Ephesians because of the influence of Paul's son in the faith, Timothy, who was a leader in Ephesus and naturally kept the edition sent to his own church -- this may also explain why Marcion preferred the Laodecian edition, and fits in with his rejection of the Pastorals
- Was, in fact, written by Timothy himself under Paul's authority, and was intended to preserve the apostle's legacy in the face of his possibly imminent death after imprisonment
Now we may consider various evidences for and against Pauline authorship/authority, in light of this framework.
External evidence. Attributions by patristic writers, by orthodox and heretical alike, are unanimous in favor of Paul. Ephesians is quoted in 1 Clement (95 AD), Ignatius, and Ploycarp, three late first century/early second century writers.
Textual evidence. If this and the patristics were all we had, the case would be open and shut. While manuscripts vary on the destination, all agree that Paul is the author.
We now move to the evidence of vocabulary and tone, and we begin by repeating some points stressed in our study of the Pastorals. As we noted there, things like choices of words should be disregarded forevermore as a determination of authorship. Word choice and writing style are NOT suitable criteria for saying that a person did or did not write a particular piece of literature - especially when we are dealing with writing samples as small as the Pastorals, or Ephesians. In this regard, conservative scholars rightly cite the work of Yule [Knig.PE, 39; Oden.12TT, 13], who notes that samples of at least 10,000 words are needed to make such determinations - and Ephesians is rather short of that mark.
What about a scribe? The odds that a scribe did most or all of Ephesians is quite high. Paul is a prisoner at this time (3:1, 4:1) and likely in chains, unable to write himself.
In this light we should outline a few basic issues about the use of scribes, and their affects on composition. As Ellis points out [Ellis.PP, 45], a trusted scribe usually shaped vocabulary, style, and composition, in direct proportion to the amount of trust the writer placed in him. E. Randolph Richards, in The Secretary in the Letters of Paul, notes that secretarial roles varied widely and included asking a secretary to send a letter to someone without specifying its contents -- only giving a broad outline of what was needed. The stated author assumes responsibility, but the secretary is the actual "author."
In such cases, the secretary was not always identified as such. Richards notes for example that Cicero asked another man, Atticus, to write letters to their various friends in Cicero's name. "Clearly Cicero intends for Atticus to write letters for him that the recipients will believe are from Cicero."  In another instance, Cicero's brother Quintus had secretaries write letters for him, which were then checked over Statius, probably his chief secretary, for problems.
If Paul was in prison, then he was probably in no condition or had no ability to do significant cross-checking, and would give his scribe considerable latitude in composition, indicating only major points to be developed -- if indeed it was someone he trusted. On this account, and given other factors, Timothy is a likely candidate. At any rate, under such compositional conditions, "the letters may have no points at all in common with the author's customary style." 
We should also share the note made in our Pastorals project from Quinn [Quin.LTi, 4], who records that early studies that confirmed the non-Paul thesis have been overturned by more a more recent (and more broadly-based) study that allows that 12 of the 13 letters ascribed to Paul (Titus is excluded!) could have been written by one very versatile author.
Word choices. Ephesians contains over 80 hapax legomena (words not found elsewhere in Paul). But 4 of these are found in the LXX, 16 outside the LXX, 38 are found elsewhere in the NT, and only 4 are otherwise unknown from the period, but are found in patristic writings. This is hardly out of line with other Pauline works, like Philippians (50 such words) or Romans (100), especially considering the amount of liturgical material.
Vocabulary usage. It is pointed out that many words used by Paul normally are used here in a non-Pauline sense. While this is often a persuasive factor, Barth does make the point that Paul sure "did not carry with him a dictionary containing inflexible definitions of key words."  Such criteria is only used, as with the end of Mark, when other evidence stands with it (in that case, textual evidence).
For that matter, not all such arguments are convincing even within Ephesians. Here are three examples cited by Barth:
Verb-noun ratio. Ephesians has a verb-noun ratio of 231:158, versus 363:377 for Romans and 139:202 for Galatians. Sentence length. The sentences in Ephesians are described as overlong and clumsy, over 3 lines long, versus 1.4 lines in Romans. Barth attributes both of these factors to, once again, the use of liturgical material in Ephesians. He notes similar methodology where Paul "employs the language of prayer and adoration." (Rom, 8:38-9, 11:33-6, and the thanksgivings at the beginning of most of his letters) He adds, "liturgical diction has a tendency toward the archaic, clumsy, and unctuous." O'Brien notes similar tendencies in Qumran psalms.
Theological differences. On this account we are served the following arguments:
Barth adds that it is our own emphasis on justification that is causing a problem here. Even in Galatians, Paul's "heaviest" justification letter, Paul enlists justification in the service of a higher unifying principle, to show that there should be no barrier between Jew and Gentile. It is unity that is his main concern, not justification, and the only difference is that in Ephesians he has no "problem" to use the justification "weapon" against, and so can go straight to the unity issue.
Similar objections are also made about lack of emphasis on the death of Christ -- the cross is mentioned only once, but it is also mentioned only twice in 1 Cor. and three times in Galatians, where it is mentioned most of Paul's letters, and never in Romans; Christ's blood is mentioned twice in Ephesians, but not at all in Galatians, and Christ's "sacrifice" is mentioned twice [5:2, 25]. On the other hand, Ephesians stresses Christ's exaltation, but so does the rest of the NT -- especially John [16:14, 17:1. etc.], which should be kept in mind.
The same verb is used in both verses. However, in Romans Paul stresses the moral foundation behind the law; here, the law is referred to in terms of an exclusive covenant, superseded by the new covenant offered by Christ. In times past, the law served as a social barrier between Jew and Gentile, giving each group separate identities; in Christ, these identities are no longer relevant, thereby forming a new community.
Some also claim that Paul's self-centeredness (3:3-7) followed by humility (3:8) seems unnatural, but this is a statement made with reference to modern personality ideas; ancient persons were far more "realistic" in their self-appraisals, and the same "tension" appears in other letters of Paul, particularly the Corinthians correspondence.
How is this contradictory? Are these assumed to be complete lists in each case? Following typical Greco-Roman rhetorical techniques, there is no way that Paul would be required to provide a complete list or one in exactly the same order every time.
A final factor appealed to is dependence on Colossians. This is an issue we will take up when we address the authorship of that letter.
In conclusion: The supreme oddity of Ephesians, and the attempt to completely remove Paul from any but post-mortem, ideological sorts of influence in its composition, lies in that even the staunchest critics admit that it is a work of genius and great spiritual insight, and not only so, but "a brilliant and comprehensive summary of Paul's theological emphases." [Mitten, 11]
In that light, one may recall the words of one of my former English professors, who said of theories that Shakespeare did not write what was attributed to him, "If he didn't write it, it was written by someone living at the same time and with the same name!" Likewise, Bruce remarks  that "of such a second Paul [who could write an Ephesians] early Christian history has no knowledge."
However, I would tender a tentative thought which ties together the evidence above, and might well explain the subtle genius and majestic beauty that lies behind Ephesians.
Ephesus is known as a place where Paul founded a church, and as Timothy's locale. But it is also tied intimately to the work of the apostle John.
In light of bits of shared vocabulary noted above, and conceptual ties (the lofty language, the focus on things heavenly, shared vocabulary with material by John, Paul, and Luke, the heavy use of liturgical material) I would suggest that our scribe for Ephesians is Timothy, and that he has been heavily influenced in his thinking not only by his father in faith, Paul, but also by the beloved apostle.
And now an update. Recently on TheologyWeb a certain Skeptical sort posted a list of 21 reasons why Ephesians is not authentic. The material was taken verbatim from another site, which in turn was merely reporting material from a seriously outdated source (none of the sources used dated later than the Depression era). Since this is stuff that is likely to be spread around, we will present answers to these 21 objections here. It will be seen that these are mostly the sort of ideas that modern, more careful scholarship has left behind. Here is the list:
. In the first place, Ephesians is unlike any of the Pauline letters known to us in that it reflects no definite historical situation which it is intended to meet. In all the efforts to interpret the letter from the Pauline point of view, no such situation has been successfully or convincingly developed. Yet Paul's letters have an unfailing way of revealing with great clearness the conditions under which they were written and the purpose in the apostle's mind. Considered as a letter of Paul's, Ephesians is in this respect altogether baffling.
This may have been baffling back in the 1930s, to certain people, but it is not any longer. Ephesians is widely understood to be a general epistle, and while there have been varying ideas as to its purpose - a theological tract, a wisdom discourse, a liturgy, or a sermon - scholarship has matured to the point where, as O'Brien puts it, there is "no reason, in principle, why a letter could not be general in nature and written for the purpose of edifying and instructing Christians over a wide area or in a range of congregations." 
The objection merely creates a problem, and then wonders why there is no solution. It is also rather vague, as it fails to show how EACH of Paul's letters "unfailingly reveals" the conditions under which they were written; in this regard, it is worthy of note that this is also a "low context" demand of a document written in a high context setting.
The writer's admiration and regard for Paul are so great as to give scholars who maintain that Paul wrote the letter great embarrassment. The representation of Paul's unique insight into Christian truth, 3:1-12, is very different from Paul's own attitude in I and II Corinthians, for example.
Once again, how this is so is not explained; and one may also compare Paul's "attitude" in Galatians 1, where Paul speaks of "unique insight" given him by Christ. Even Best, who argued against Pauline authorship, agreed that the picture Paul painted of himself is consistent with his other letters, allowing for some development Paul was perfectly capable of himself.
Furthermore, the entire premise of this objection is rooted in a Western values system, in contrast to the emphasis on openness and frankness of speech found esteemed as a value in the ancient world (as I have noted elsewhere, Pilch and Malina's Handbook of Biblical Social Values notes that in the ancient world, the qualities of boldness, openness, frankness, and self-confidence were highly valued, and though often a privilege limited to the upper class, was approached as a "golden mean" that kept one from on the one hand "being impudent before equals" and on the other hand "fawning before superiors."
Finally of course, if Timothy wrote under Paul's authority as we hold, the problem disappears anyway.
With this goes also the writer's veneration for the holy apostles and prophets as the foundations of the church, 2:20, and the mediums of revelation, 3:5 Paul thought of Christ as the foundation of the church and said there could be no other, I Cor. 3:11. The Gospel of Matthew is usually understood as speaking of the apostle Peter (or perhaps his messianic faith) as the rock on which the church was to be built. Matt. 16:18. The expression "holy prophets" recalls Luke 1:70, and the conception of the apostles as the foundation of the church reminds us of the heavenly city in the Revelation, the wall of which had twelve foundation stones, and on them were the twelve names of the Lamb's twelve apostles, 21:14; compare also Rev. 18:20. The grouping of holy apostles and prophets is hardly the way in which Paul, as we know him through his letters, would have expressed himself. In fact, it points to an attitude to the apostles more like that of Matthew, Revelation, and Luke-Acts than of Paul.
To begin, the word on 1 Cor. 3:11 is somewhat exaggerated; there Paul says, "For other foundation can no man lay than that is laid, which is Jesus Christ," which only implies at most that no man can lay another foundation, not that another cannot be laid (as by God, for example); and anyway, Paul speaks of a foundation for a person's life ("if any man", 3:12), and specifically, of how one as a missionary plants a church (3:10), not of the foundation for the church's organizational structure.
The Gospel quotes merely confirm this point: The apostles may indeed - as founding members of the Jesus movement - be regarded as the "foundation" of the ekklesia (the household of God) in the sense that a collectivist society would recognize, and they were chosen by Jesus, who therefore laid this foundation - not men.
The church, in Ephesians, is always the church universal, never the individual local church; for that Ephesians seems to use patria,  the equivalent of familia, 3:15. But Paul uses "church" (ecclesia) in both senses. Gal. 1:2; I Cor. 16:19.
This would again make sense if Ephesians were an encyclical; however, the objection itself is the product of an individualist orientation. Ancients would regard their ingroup as the "ingroup universal" even as Diaspora Jews never thought of themselves primarily in terms of "the individual local synagogue".
. The church has become Greek; for the whole body of Christians addressed in 1:1 were once physically heathen, 2:2, 11. There is no room for any Jewish Christianity in the picture.
Of course since the matter of there being converted Jews in the church would have obtained at any point in Christian history for the first 100-200 years, this is an "issue" whether Paul was the author or not. That said, it escapes the imagination that the meaning of 2:11 (2:2 is not as clear), when it says, "remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands," it is as to say, "anyone out there who happens to be Gentile, hearing this - you who were Jews, this does not apply to you, implied."
Paul's readers would be predominantly Gentile for this letter, and it is too demanding to expect him to add diversionary footnotes just to solve our unnecessary confusion.
The writer himself had been in the same condition, 2:3, and hence is a gentile Christian. Paul scrupulously distinguished between the sins of the lews and the grosser ones of the heathen, Romans, chapters 1, 2. It is these grosser ones which the writer now confesses for himself and his readers. Both he and they are Greek; compare II Cor. 11:22; Gal. 2:15; Phil. 3:4.
As already noted, 2:2 is not at all clear in terms of what is stated previously; 2:2-3 says, Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience: Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.
Clearly the matter is not one of "being a Greek" but of being a sinner, which Jews and Gentiles alike were, to some level (even as admitted). That said, note the shift from "ye" in 2:2 to "we all" in 2:3, indicating a shift in topic from one group (Gentiles) to another, expanded group (all people, including Jews and Gentiles).  Thus the argument fails either way.
The encyclical form of the letter (that is, with no place name in 1:1), well known from the text of the great fourth-century manuscripts Vaticanus and Sinaiticus, and recently confirmed by the Ann Arbor-Chester Beatty papyrus, ca. A.D. 200, clearly implies a conception of Paul as a notable letter-writer. No one would put out an encyclical letter in the name of a man who had written no letters to speak of. But this writing of an encyclical implies that the letters of Paul have been collected, so that the writer can think of him as a notable letter-writer.
An encyclical implies nothing more than that Paul was a 1) literate man 2) held in high esteem and honor by a great many churches, which is already obviously true from his letters regarded as authentic. It is too demanding to suggest that Paul needed to have a specific reputation as a letter-writer for an encyclical to be done, and an arbitrary step beyond that to suggest that the letters had to be collected before an encyclical could be done.
The sects are already beginning to appear, as in the Acts 20:30 and the Revelation, 2:6,15. Christians "must not be babies any longer, blown about and swung around by every wind of doctrine, through the trickery of men with their ingenuity in inventing error," 4:14. This is the meaning of the insistence on unity in 4:3-6.
One is bewildered by the pretense that one needs to wait any long amount of time for sects to appear. How long did it take for Israel to worship the Golden Calf? In syncretistic pagan Rome, it would be more surprising if sects appeared later rather than sooner.
While so much of the language is Paul's own, it is used in other senses than Paul's. The "secret" of Col. 1:27 is Christ in the believer; in Ephesians the "secret" is the enfranchisement of the heathen as of equal rights with the Jews in the Christian salvation, 3:6. The "principalities and dominions" that the Colossians were tempted to worship (Col. 1:16; 2:10) have in Ephesians become the spiritual enemies with whom the Christian soldier has to grapple, 1:21; 6:12.
One is hard pressed to see how these usages are mutually exclusive, and it is not shown that or how they are. It is not as though God had only one secret or mystery to reveal, or as if any matter has exclusive rights to the word; and the position of the principalities and powers with relation to the believer verses his former state is a perfectly natural evolution.
The style is reverberating and liturgical, not at all the direct, rapid, Pauline give-and-take. For example, the Spirit, or the Spirit of God, or the holy Spirit, becomes "the holy Spirit of God," 4:30.
This may be resolved if Timothy is the author. However, as O'Brien notes  the unique structure serves a purpose, for it "emphatically underscores the identity of the one who may be offended, and thus the seriousness of causing him distress." Paul is obviously as capable of this sort of literary artistry as the anonymous genius critics suppose are behind Ephesians.
. The novel element in the vocabulary, that is the words used in Ephesians but not found in the nine genuine letters, is mostly akin to works like Luke-Acts, I Clement, I Peter, and Hebrews, written toward the close of the century.
Since no specifics are offered on this, no comment is possible, but we have noted a general answer above.
The interest in hymnology, illustrated by the quotation of a Christian hymn, 5:14, points to the time when that interest had begun to be active in the early church, as in the canticles of Luke, 1:42, 46, 68; 2:14, 29, and the arias/choruses, and antiphonies of the Revelation.
Jews of the synagogue had been singing psalms and hymns long before Jesus walked the earth; there was no need for "interest" in hymns to be planted, and hymns and liturgies are also seen in genuine Paulines (e.g., Phil. 2:6-11). In addition, Mormons for example were writing specially LDS hymns within 30 years of Joseph Smith.
The Descent into Hades, 4:9, 10, is an extraordinary thing to have to fit into the Pauline theology and is generally passed over in silence by those who seek to reconstruct Paul's thought.
I cut this one short because it is erroneous from the get-go. Ephesians does not teach a descent into hell; see link below.
This brings us to the use of the Acts in Ephesians. The writer of Ephesians not only thinks of Paul as where the Acts left him, a prisoner for the Greek mission, but in several instances reflects the language of Luke-Acts, while the Descensus doctrine of Ephesians seems to be an inference from the Ascension doctrine of Luke.
Without specifics of this alleged connection, this is again impossible to answer, but either Luke as a companion to Paul, or Timothy as a writer, explains the language matter. There is of course nothing unintelligible about Paul having written Ephesians while under house arrest in Rome.
The reference to the breaking-down of the barrier that kept the heathen out of the Court of the Men of Israel in the Temple, 2:14, while of course figurative, is certainly more natural after the Temple had been destroyed in A.D. 70 than before.
"More natural" is a subjective value judgment, not an objective data point. See above, however, where we priorly dealt with this.
The jubilant review of the blessings of the Pauline salvation, with which the letter begins, is more natural in a reader of Paul's letters than in Paul himself. Paul's way was to take one of these and dwell upon it. But in chapter 1 they fairly tumble over one another, with no full treatment of any of them. This is entirely natural if the collected letters of Paul, with full treatments of all the matters so tumultuously surveyed in chapter 1, followed Ephesians.
Once again, "more natural" is a mere subjective judgment. It is just as well to say it is "natural" if Paul (or Timothy) is writing a generalized encyclical summarizing the blessings of salvation.
The injunction in Ephesians 6:4 to bring their children up with Christian training and instruction is out of keeping with Paul's attitude; all he had to say to parents in Colossians (written supposedly at the same time as Ephesians, if the latter is by Paul) was "Do not irritate your children," 3:21.
This one is quite unintelligible as it stands; not only is it not explained how 6:4 is "out of keeping" (as if Paul had to say everything exactly the same way, each time), it fails to explain how there is any mutual exclusivity between bringing up children with Christian training, and not irritating them.
Ephesians as a whole is a generalization of Paulinism much more like a later Paulinist than like Paul himself. Someone has observed that it reads like a commentary on Paul. Even Romans is less of a generalization of the Pauline positions than Ephesians.
This too is too vague to warrant a response. It offers no specifics and so cannot be replied to.
The writer of Ephesians is far more of an ecclesiastic than Paul. He finds in the church a great spiritual fellowship, built upon the apostles and prophets, 2:20-22, the medium of God's revelation 3:10, and the avenue of man's praise, 3:21. It is the bride of Christ, 5:25-32, compare Rev. 21:9, 10.
Of course this would make perfect sense of ecclesiology is Paul's main subject; this is like comparing a scientist's essay on genetics to a letter he wrote to his wife and saying, "This letter can't be by the scientist, for the author of the letter is far less of a geneticist than the author of the essay."
It is amazing how scholars in the past (and even today) have been neglectful of the point that men write on various topics, and that topics control substance. And as noted, the collective nature of the ancients means that the principle of a "great fellowship" was already part of the mindset.
Ephesians shows the literary influence of every one of the nine genuine letters. Over and over again it reveals acquaintance with each one of them. Every Pauline letter displays something in common with one or more of the others, of course. But Ephesians shows knowledge of all the other nine-a state of things which cannot be matched or even approached in any other letter. To some this seems proof that Paul himself wrote Ephesians. But elsewhere Paul never repeats himself to any such extent as this. Moreover, the writer uses these Pauline materials to build up something very different from Paul.
Like several above, this offers no specifics and is too vague to answer. I will say that I have not seen such an argument in any modern commentary, though arguments are often made concerning likenesses to Colossians. The 21st and last point is merely an expansion on this one and does not need to be treated specially.