10 In this way, then, as Matthew undertook the task of presenting the record of Christ as the King who came into this world, and into this earthly and mortal life of men, for the purpose of exercising rule over us who have to struggle with temptation, he began with Abraham, and enumerated forty men. For Christ came in the flesh from that very nation of the Hebrews with a view to the keeping of which as a people distinct from the other nations, God separated Abraham from his own country and his own kindred. And the circumstance that the promise contained an intimation of the race from which He was destined to come, served very specially to make the prediction and announcement concerning Him something all the clearer.
Thus the evangelist did indeed mark out fourteen generations in each of three several members, stating that from Abraham until David there were fourteen generations, and from David until the carrying away into Babylon other fourteen generations, and another fourteen from that period on to the nativity of Christ. But he did not then reckon them all up in one sum, counting them one by one, and saying that thus they make up forty-two in all. For among these progenitors there is one who is enumerated twice, namely Jechonias, with whom a kind of deflection was made in the direction of extraneous nations at the time when the transmigration into Babylon took place. When the enumeration, moreover, is thus bent from the direct order of progression, and is made to form, if we may so say, a kind of corner for the purpose of taking a different course, what meets us at that corner is mentioned twice over—namely, at the close of the preceding series, and at the head of the deflection specified.
And this, too, was a figure of Christ as the one who was, in a certain sense, to pass from the circumcision to the uncircumcision, or, so to speak, from Jerusalem to Babylon, and to be, as it were, the corner-stone to all who believe in Him, whether on the one side or on the other. Thus was God making preparations then in a figurative manner for things which were to come in truth. For Jechonias himself, with whose name the kind of corner which I have in view was prefigured, is by interpretation the “preparation of God.” In this way, therefore, there are really not forty-two distinct generations named here, which would be the proper sum of three times fourteen; but, as there is a double enumeration of one of the names, we have here forty generations in all, taking into account the fact that Christ Himself is reckoned in the number, who, like the kingly president over this [significant] number forty, superintends the administration of this temporal and earthly life of ours.
Source: The Harmony of the Gospels (New Advent)