Maimonides on Genealogical Purity

בימי המלך המשיח, כשתתיישב מלכותו ויתקבצו אליו כל ישראל, יתייחסו כולם על פיו ברוח הקודש שתנוח עליו, שנאמר "וישב מצרף ומטהר . . ." (מלאכי ג,ג). ובני לוי מטהר תחילה, ואומר זה מיוחס כוהן וזה מיוחס לוי, ודוחה את שאינן מיוחסין לישראל: הרי הוא אומר "ויאמר התרשתא להם . . . עד עמוד כוהן, לאורים ולתומים" (עזרא ב,סג). הנה למדת שברוח הקודש מתייחסין המוחזקין, ומודיעין המיוחס. ואינו מייחס ישראל אלא לשבטיהם, שמודיע שזה משבט פלוני וזה משבט פלוני. אבל אינו אומר על שהן בחזקת כשרות, זה ממזר וזה עבד--שהדין הוא שמשפחה שנטמעה, נטמעה.

(רמב"ם הלכות מלכים ומלחמות יב:ו (ג-

In the days of the King Messiah, when his dominion is stabilized and all of Israel gathers to him, the genealogy of all will be established by his word, through the holy spirit (ru’ach ha-kodesh) that will rest upon him, as it says (Malachi 3:3) “and he shall sit as a refiner and purifier…”. The Levites will be purified first, and he will say “this one is a pedigreed Kohen, this one is a pedigreed Levite”, and he will drive all non-pedigrees into [the general category of] the Israelites. Behold, it says (Ezra 2:62-63) “[These sought their register, that is, the genealogy, but it was not found; therefore were they deemed polluted and put from the priesthood.] And the governor said unto them, that they should not eat of the most holy things, till there stood up a priest with the Urim and Tumim.” Thus we learn that through the holy spirit the presumed will be pedigreed and the pedigreed will be identified. And he will only trace Israelites to their tribe; he will make known that this person is from Tribe X and that person is from Tribe Y. But he will not say about those who are presumed to be kosher, “This one is a bastard; that one is a slave”, since the Halakha is that a family that commingled, has commingled.

-Maimonides, Laws of Kings and Wars, 12:6(3)

[On the Topic of Rambam and his generally non-essentialist approach to personal and Jewish identity, see Greg’s excellent post. It also sheds new light on Rambam’s famous expansion of ‘the Tribe of Levi’ to include anyone who devotes themselves exclusively to Torah in Laws of Shemittah and Yovel 13:11(13). And see here, especially toward the end, and see also the paper by Menachem Kellner entitled "Philosophical Misogyny in Medieval Jewish Thought: Gersonides vs. Maimonides". (I don’t have the article; I heard it from him as a lecture).]