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"A bastard shall not enter into the assembly of Yahweh; even to the tenth generation shall none of his enter into the assembly of Yahweh" (Dt. 23:2).
And yet, it is said, David was a ninth-generation descendant of Perez, the bastard son of Judah and Tamar (Gen. 38:24-30 ; Ruth 4:18 ; 1 Chron. 2:5-14 ) and was allowed into the assembly.
The word the KJV uses for "bastard" is mamzer -- a word used only twice in the NT, here and in Zech. 9:6. This word comes from a root meaning "alienate" and is used in the sense of a mongrel, i.e., according to Strong's, "born of a Jewish father and a heathen mother."
In light of this context, Tamar wasn't married to Judah, true enough, but for this to be an issue, she would have had to be a heathen -- and we see no indication of that in the text.
It may be said that "mamzer" has other meanings: A person born of adultery, a person born of incest, or a person born of a forbidden or mixed marriage." On the latter David is said to still be out because his ancestress Ruth was Moabite; see link at end for a partial response.
But generally: As a female, Ruth was able to be absorbed into the Jewish nation (due to the principle of patrilineal descent) and as a believer in the true God was no more considered a "heathen" of the sort forbidden for marriage under the definition of mamzer.
Perhaps this is enough for some, but in an article entitled "What is the Biblical meaning of 'mamzer'?" on askarabbi.com, Rabbi Michael Samuel offers these observations:
Among modern Hebraic scholars, it has been argued that the Deut reference also refers to a child of mixed parentage‹Hebrew and pagan as in Neh. 13:23-25:"At that time too, I saw Jews who had married wives from Ashdod, Ammon and Moab; as regards their children, half of them spoke the language of Ashdod or the language of one of the other peoples, but could no longer speak the language of Judah. . . " I think that it is more logical to say that the term "mamzer" may have originally referred to a mixture of different ethnic peoples. Mamzer may also refer to the name of an ethnic group or a term for an ethnic group which lived among the Philistines. (Cf. Tigay's excellent commentary on Deu. 23:3).
Rashi notes on this verse to mean "And the strangers shall dwell in Ashdod" ‹ And a strange people shall dwell in Ashdod. Those are the Israelites, who were strange in it. However, Ibn Ezra cites a Rabbi Yehudah ben Bilam who wrote that mamzer simply refers to a "name of a nation" but he differs. The term mamzer refers here to one born of incest or anyone who would be guilty of the sin of excision. These people of unfit lineage will be separated from the rest of the "fit" Israelites, who would inhabit the Philistine cities. Radak in his notes, also cites the view that the term "mamzer" according to some refers to the name of a non-Israelite tribe but like Ibn Ezra, argues in defense of the traditional rabbinical and further suggests that the people of Israel isolated all those who were Halachically deemed "illegitimate." This interpretation is forced and there is no evidence from the text that this was ever the case.
In defense of Yehudah ben Bilam's reading, the text in Zechariah 9:6 may mean that all the cities of Israel and the city of Ashdod, the principle city where the Philistines dwelled, would be deserted except that there would be there a few scattered and wandering inhabitants, like those who sojourn in a strange land. Another ethnic people would come to dwell there just as other ethnic peoples later came to dwell in Israel after the destruction of her capital. In the new Da'at Mikra commentary produced by Bar Ilan University, we find a similar interpretation to the one proposed by Rabbi Yehuda ben Bilam makes a good case that the mamzerim were originally a group of Canaanite peoples much like the Netinim were; the mamzerim were originally a mixture of different foreign nations. Several scholars R. Yehuda Keil, Professors Aharon Mirski, Eliezer Alinar, and Feivel Meltzer each subscribed to this interpretation.
When we examine this passage in conjunction with the passage found in Deut. 23:3-6, an interesting pattern emerges:
"No one whose testicles have been crushed or whose penis has been cut off may be admitted into the community of the LORD. No child of an incestuous union may be admitted into the community of the LORD, nor any descendant of his even to the tenth generation. No Ammonite or Moabite may ever be admitted into the community of the LORD, nor any descendants of theirs even to the tenth generation, because they would not succor you with food and water on your journey after you left Egypt, and because Moab hired Balaam, son of Beor, from Pethor in Aram Naharaim, to curse you . . . . .Children born to them may in the third generation be admitted into the community of the LORD" (Deu. 2:2-9).
The text in Deuteronomy may be speaking about those outsiders who might want to enter through marriage and conversion, into the Israelite people. Those men who allowed themselves to be castrated for idolatry, are barred from entry. A foreign people referred to as "mamzer" was also included, as were the Amonites, Moabites. Even the Egyptians were allowed entrance by the third generation.