How can we understand the fact that our Mashiach, King David descends from sullied and problematic lineage? His paternal ancestors, including his great grandfather Boaz, were the result of an act of incest between Yehuda and his daughter-in-law Tamar. She deceived Yehuda by posing as a harlot in order to have his child. Yet Yehuda is the father of the tribe from whom the Mashiach will emerge (Bereshit 38-39, Ruth 4).

On King David's maternal side, was a convert to Judaism, Ruth. She was a Moabite princess, and although the Torah prohibits Moabite converts into the Jewish People (Devarim 23), the Beth Din of Boaz ruled that this prohibition applies only to male Moabites and not to females. Moreover, Moab, Ruth's ancestor, was the result of incest between Lot and his daughter (Bereshit 19). Therefore, Ruth, the great grandmother of the Mashiach, descends from a very questionable past.

What is Judaism teaching by having the Mashiach descend from incestuous acts? The Midrash states that this Messianic ancestry was purposely designed, the proof-text being the strange use of the word ZERA (seed) rather than BEN (son), in reference to Boaz and Lot.

When Boaz married Ruth, the Jews blessed the couple at the gates of the city of Efrat, saying, "May your house become like the house of Peretz, whom Tamar bore to Yehuda, from the ZERA (seed) which G-D gives you from this NA'ARA, (young woman)" (Ruth 4).

It is strange for the text to refer to Ruth, a widow for at least a decade as a NA'ARA, and the reference to Lot's act of incest with his daughter which seems like a bizarre blessing. Based on this verse in Ruth, the Midrash interprets the verse in Bereshit 19, "And the elder daughter said to the younger, 'Come let us make our father drunk with wine and let us be with him, so that we may enable our father to give life to his ZERA'." The Midrash continues that this ZERA in the Book of Ruth is the same ZERA that comes from Lot and his daughter (Midrash Ruth Zuta).

The idea that good can emerge from evil is built into the Jewish concept of the Mashiach. Just as the grand-daughter of the cruel and immoral Moab could become the loving, modest, and gracious Ruth, all people of despair can rehabilitate themselves and bring the Mashiach.

Rav Yosef Soloveitchik understood this idea from this Midrash. The Rav explains that this idea is precisely the lesson that Lot's daughter wanted to impress on her younger sister in Bereshit 19, "The elder sister said to the younger, 'Our father is old and there is no man on earth to come to us in the manner of all societies.'" The Midrash Bereshit Rabba states that these sisters believed the entire world had been destroyed as in the days of Noach's Flood.

Thus, Rav Soloveitchik explains, the elder sister suggested that they each deceive and seduce their father through intoxication, so that they could repopulate the earth. The younger sister hesitates at the act of incest, and she sees no point in attempting to restart the world. After all, G-D attempted to establish a perfect world, first in Eden with Adam and Chava and again with Noach and the Covenant of the Rainbow (Bereshit 9). Both ended in failure, for humanity sank repeatedly into immorality and corruption. This sister felt it would be absurd and in this case of incest, immoral, to begin humanity once again.

The elder sister would not give up said Rav Soloveitchik. She argued that G-D would never have created the human being in the Divine Image if evil were to triumph, and if human civilization would destroy itself.

No, she insisted, we must have faith in the possibility of Teshuva. As the verse in Mishlei states, "The Tzaddik will fail and fall 7 times, but he keeps rising up". The Baal Shem-Tov explains this verse as ONLY after a person fails and falls 7 times, and doesn't give up, but continues to try to better himself, only then is he a Tzaddik.

Belief in the Mashiach is based on faith in the power of Teshuva. This is the lesson that Lot's daughter was teaching humanity, Therefore, she became the ancestor of the Mashiach, who will redeem all of humanity.