So all the generations from Abraham to David are fourteen generations; and from David until the carrying away into Babylon are fourteen generations; and from the carrying away into Babylon unto Christ are fourteen generations.
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Theophylact of Ochrid
He divides the generations into three [different] conditions [of leadership], to show the Jews that although they were ruled by judges, as they were until David, and by kings, as they were until the deportation, and by priests, as they were until Christ, yet it did not benefit them at all in acquiring virtue; but they were in need of the true Judge and King and Priest, Who is Christ. For when the line of their rulers had failed, then Christ came, in accordance with the prophecy of Jacob. How can there be fourteen generations from the deportation to Babylon until Christ when only thirteen persons are mentioned? If the reckoning of ancestry through the mother could be given, we would list Mary as well, and thus complete the number. But since it cannot, how can this be resolved? Some say that he counted the deportation itself as a person [i.e. as a generation].