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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Augustine of Hippo
De Cons. Evan, ii, 4: Or, one of Christ’s forefathers is counted twice, because in him, Jeconiah to wit, there was made a passing off to strange nations since he was carried to Babylon. Wherever a series turns out of the right line to go in any other direction there is an angle made, and that part that is in the angle is reckoned twice. Thus here is a figure of Christ, who passes from the circumcision to the uncircumcision, and is made a cornerstone.
After having divided the whole into three periods of fourteen generations, he does not sum them all up and say, The sum of the whole is forty and two; because one of those fathers, that is Jeconiah, is reckoned twice; so that they do not amount to forty-two, as three times fourteen does, but because one is reckoned twice over, there are only forty-one generations. Matthew therefore, whose purpose was to draw out Christ’s kingly character, counts forty successions in the genealogy exclusive of Christ. This number denotes the time for which we must be governed by Christ in this world, according to that painful discipline which is signified by the iron rod of which it is written in the Psalms, “Thou shalt rule them with a rod of iron. "That this number should denote this our temporal life, a reason offers at hand, in this, that the seasons of the year are four, and that the world itself is bounded by four sides, the east, the west, the north, and the south. But forty contains ten four times. Moreover, ten itself is made up by a number proceeding from one to four.