The Descent of a Lofty Soul - Moshiach

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And they gave their father wine to drink on that night, and the elder came and lay with her father, and he did not know of her lying down or of her rising up -  וּבְקוּמָֽהּ

According to the Zohar, the dot in the הּ in the word וּבְקוּמָֽהּ [Vayera 19:33] alludes to the fact that G-d was secretly assisting this event, because Moshiach was to materialize from it [since Ruth, King David's great grand-mother, was a convert from the Moabites].  The latter event, between Lot and his younger daughter [19:35] is written without the letter vav - וּבְקֻמָֽהּ - to indicate that the union did not produce such great offspring.   Rabbi Shimon said "When the verse states that Lot ''wasn't aware'', it means he wasn't aware that Moshiach was destined to come from this union" [Zohar I 110b]

Why should the beginnings of Moshiach occur in such an undignified manner?  Ramak explains that when a very lofty soul is about to descend into the world, the forces of kelipah [“peel” or “shell”—words used by Kabbalah to describe layers of impurity] oppose the soul's descent vehemently. Sometimes, however, kelipah will consent to the soul's descent if it occurs amidst a particularly sinful act.  Thus, we find that from this undesirable act, the ancestor of Moshiach was born.

Source: Lubavitcher Rebbe