15 The carrying away into Babylon took place of old by Jechonias, who was not permitted to reign in the nation of the Jews, as a type of Christ, whom the Jews would not have reign over them. Israel passed over unto the Gentiles, that is, the preachers of the Gospel passed over unto the people of the Gentiles. What marvel then, that Jechonias is reckoned twice? For if he were a figure of Christ passing over from the Jews unto the Gentiles, consider only what Christ is between the Jews and Gentiles.
Is He not that Cornerstone? In a corner-stone you see the end of one wall, and the beginning of another; up to that stone you measure one wall, and another from it; therefore the corner-stone which connects both walls is reckoned twice. Jechonias then as prefiguring the Lord was, as it were, a type of the corner-stone; and as Jechonias was not permitted to reign over the Jews, but they went unto Babylon, so Christ, “the stone which the builders rejected, is made the head of the corner,” that the Gospel might reach unto the Gentiles.
Hesitate not then to reckon the head of the corner twice, and you have at once the number written: and so there are fourteen in each of the three divisions, yet altogether the generations are not forty-two, but forty-one; for as when the order of the stones runs in a straight line, they are all reckoned but once, but when there is a deviation from the straight line to make an angle, that stone at which the deviation begins must be reckoned twice, because it belongs at once to that line which is finished at it, and to that other line which begins from it; so as long as the order of the generations continued in the Jewish people, it made no angle in the regular division of fourteen; but when the line was turned that the people might pass over into Babylon, a sort of angle as it were was made at Jechonias, so that it was necessary to reckon him twice, as the type of that adorable Cornerstone.
Source: Sermons on Selected Lessons of the New Testament (New Advent)