III. The facts of the Gospel attest the Incarnation.
The true birth of Christ, therefore, is confirmed by the true cross; since He is Himself born in our flesh, Who is crucified in our flesh, which, as no sin entered into it, could not have been mortal, unless it had been that of our race. But in order that He might restore life to all, He undertook the cause of all and rendered void the force of the old bond, by paying it for all, because He alone of us all did not owe it: that, as by one man's guilt all had become sinners, so by one man's innocence all might become innocent, righteousness being bestowed upon men by Him Who had undertaken man's nature. For in no way is He outside our true bodily nature, of Whom the Evangelist in beginning his story says, “the book of the generation of Jesus Christ, the son of David, the son of Abraham,” with which the blessed Apostle Paul's teaching agrees, when he says “whose are the fathers and of whom is Christ according to the flesh, Who is above all God blessed for ever,” and so to Timothy “remember,” he says, “that Jesus Christ has risen from the dead, of the seed of David.”
Source: Letters (New Advent)