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In Matt. 27:9-10, more than one prophet is cited in a quote; yet only one is mentioned by name. In 2 Chron. 36:21, the first part of the verse is drawn from Lev. 26:34-35, the second is from Jer. 25:12, yet only Jeremiah is listed.
What does this tell us? That it was an accepted practice to list the prophet who was making the main point. Composite attributions suit a common practice of Jewish exegetes. Z. H. Chages in The Student's Guide to the Talmud [172ff] relates a practice of the rabbis of quoting various persons under one and the same name. The rabbis "adopted as one of their methods that of calling different personages by one and the same name if they found them akin in any feature of their characters or activities or if they found a similarity between any of their actions."
Thus for example Malachi and Ezra are said to be the "same person" (Meg. 15a) because they both say similar things (Mal. 2:2, Ez. 10:2). Chages gives examples of as many as three people being treated as one person because of such similarities.
The purpose of this collapsing down of identifies was to enact a principle of praising the righteous and pious, and honoring those due such praise. Thus when Mark attributes the words of Malachi to Isaiah, he is enacting this principle by essentially melding the two prophets and giving attribution to the one who is the most deserving of honor and praise.
A reader sent me this from Noel Weeks, PhD in ancient history and languages at Brandeis University under Cyrus Gordon, as it appeared in Australian Presbyterian, February, 2009, which sums it up well:
Let me tell you a story about when I was doing my PhD at Brandeis University in Boston. Brandeis is a leading Jewish university. I remember sitting in a lecture by a very fine Jewish scholar, Nahum Sarna, who was talking about the canon of the Old Testament as it was understood in early Judaism. One of the topics he touched on was the order of the books. He said, "Well, you know that there was a period in which Jeremiah was regarded as the first book of the prophets."
Of course, nobody in the class knew that. Anyway, he continued, "One proof is that you have a quote from Zechariah quoted as being from Jeremiah because in the Jewish way of labelling things you call a book by its first few words, and you call a collection of books by the first book in that collection. Thus one of the evidences that we have of Jeremiah being the first book of the prophets in the first century is the New Testament." I was sitting there thinking, "This Jewish audience doesn’t understand why that’s an important question, because this particular text has been held up as proof that there are errors in the New Testament. All it says is that the New Testament is a Jewish document. It is speaking in the language that Jews would speak and understand."