It is written (Gal 4:4): “When the fullness of time was come, God sent His Son,” concerning which the gloss says that “the fullness of time – is when it was decreed by God the Father to send his Son.” But God decreed everything by His wisdom. Therefore, God became incarnate at the most fitting time; and it was not fitting that God should become incarnate at the beginning of the human race.

Since the work of the Incarnation is principally ordained to the restoration of the human race by the removal of sin, it is manifest, that it was not fitting for God to become incarnate at the beginning of the human race before sin. For medicine is given only to the sick. Hence, Our Lord Himself says (Matt. 9:12): “They that are in health need not a physician, but they that are ill…For I am not come to call the just, but sinners” to repentance.

Nor was it fitting that God should become man –immediately – after sin.

First, on account of the manner of man’s sin, which was the result of pride; hence, man was to be liberated in such a manner that he might be humbled, and see how he stood in need of a deliverer. Hence, on the words in Gal. 3:19: “Being ordained by angels in the hand of a mediator,” the gloss says: “With great wisdom was it so ordered that the Son of Man should not be sent immediately after man’s fall. For first of all God left man under the natural law, with the freedom of his will, in order that he might realize his natural strength; so that, having recognized his infirmity, he might cry out for a physician, and beg the help of grace.”

Secondly, on account of the order of furtherance in good, whereby we advance from imperfection to perfection. Hence, the Apostle says (1 Cor. 15:46): “Yet that was not first which is spiritual, but that which is natural; afterwards that which is spiritual…The first man was of the earth, earthly; the second man from heaven, heavenly.

Thirdly, on account of the dignity of the Incarnate Word, for on the words (Gal. 4:4): “But when the fullness of the time has come,” the gloss says: : “The greater the Judge Who was coming, the more numerous was the band of heralds who ought to have preceded Him.”

Fourthly, lest the fervor of faith should cool by the length of time, for the charity of many will grow cold at the end of the world. Hence (Luke 28:8), it is written: “But yet the Son of Man, when he comes, shall he find, think you, faith on earth?”

Charity does not put off bringing assistance to a friend; for always keep in mind the circumstances as well as the condition of the persons. For if the physician were to give the medicine at the very outset of the ailment, it would do less good, and would hurt rather than benefit. And hence, our Lord did not bestow upon the human race the remedy of the Incarnation in the beginning, lest they should despise it through pride, if they did not already recognize their disease. (Summa 3a, q.1, a. V).