Objection 1. It would seem that it was fitting that God should become incarnate in the beginning of the human race. For the work of the Incarnation sprang from the immensity of Divine charity, according to Ephesians 2:4-5: "But God (Who is rich in mercy), for His exceeding charity wherewith He loved us... even when we were dead in sins, hath quickened us together in Christ." But charity does not tarry in bringing assistance to a friend who is suffering need, according to Proverbs 3:28: "Say not to thy friend: Go, and come again, and tomorrow I will give to thee, when thou canst give at present." Therefore God ought not to have put off the work of Incarnation, but ought thereby to have brought relief to the human race from the beginning.
Objection 2. Further, it is written (1 Timothy 1:15): "Christ Jesus came into this world to save sinners." But more would have been saved had God become incarnate at the beginning of the human race; for in the various centuries very many, through not knowing God, perished in their sin. Therefore it was fitting that God should become incarnate at the beginning of the human race.
Objection 3. Further, the work of grace is not less orderly than the work of nature. But nature takes its rise with the more perfect, as Boethius says (De Consol. iii). Therefore the work of Christ ought to have been perfect from the beginning. But in the work of Incarnation we see the perfection of grace, according to John 1:14: "The Word was made flesh"; and afterwards it is added: "Full of grace and truth." Therefore Christ ought to have become incarnate at the beginning of the human race.
On the contrary, It is written (Galatians 4:4): "But when the fulness of the time was come, God sent His Son, made of a woman, made under the law": upon which a gloss says that "the fulness of the 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.
I answer that, Since the work of Incarnation is principally ordained to the restoration of the human race by blotting out 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 (Matthew 9:12-13): "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."
Nor was it fitting that God should become incarnate immediately after sin. First, on account of the manner of man's sin, which had come 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 Galatians 3:19, "Being ordained by angels in the hand of a mediator," a 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 know his natural strength; and when he failed in it, he received the law; whereupon, by the fault, not of the law, but of his nature, the disease gained strength; so that having recognized his infirmity he might cry out for a physician, and beseech the aid of grace."
Secondly, on account of the order of furtherance in good, whereby we proceed from imperfection to perfection. Hence the Apostle says (1 Corinthians 15:46-47): "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, earthy; the second man from heaven, heavenly."
Thirdly, on account of the dignity of the incarnate Word, for on the words (Galatians 4:4), "But when the fulness of the time was come," a 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 18:8) it is written: "But yet the Son of Man, when He cometh, shall He find think you, faith on earth?"
Reply to Objection 1. Charity does not put off bringing assistance to a friend: always bearing in mind the circumstances as well as the state 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 the Lord did not bestow upon the human race the remedy of Incarnation in the beginning, lest they should despise it through pride, if they did not already recognize their disease.
Reply to Objection 2. Augustine replies to this (De Sex Quest. Pagan., Ep. cii), saying (2) that "Christ wished to appear to man and to have His doctrine preached to them when and where He knew those were who would believe in Him. But in such times and places as His Gospel was not preached He foresaw that not all, indeed, but many would so bear themselves towards His preaching as not to believe in His corporeal presence, even were He to raise the dead." But the same Augustine, taking exception to this reply in his book (De Perseverantia ix), says: "How can we say the inhabitants of Tyre and Sidon would not believe when such great wonders were wrought in their midst, or would not have believed had they been wrought, when God Himself bears witness that they would have done penance with great humility if these signs of Divine power had been wrought in their midst?" And he adds in answer (De Perseverantia xi): "Hence, as the Apostle says (Romans 9:16), 'it is not of him that willeth nor of him that runneth, but of God that showeth mercy'; Who (succors whom He will of) those who, as He foresaw, would believe in His miracles if wrought amongst them, (while others) He succors not, having judged them in His predestination secretly yet justly. Therefore let us unshrinkingly believe His mercy to be with those who are set free, and His truth with those who are condemned." [The words in brackets are not in the text of St. Augustine].
Reply to Objection 3. Perfection is prior to imperfection, both in time and nature, in things that are different (for what brings others to perfection must itself be perfect); but in one and the same, imperfection is prior in time though posterior in nature. And thus the eternal perfection of God precedes in duration the imperfection of human nature; but the latter's ultimate perfection in union with God follows.
Source: Summa Theologica (New Advent)