Thomas Aquinas, Compendium of Theology

As was mentioned in the preceding chapter, the humanity of Christ is related to His divinity as a sort of organ belonging to it. The disposition and quality of organs are gauged chiefly by the purpose, though also by the dignity, of the person using them. Consequently we are to esteem the quality of the human nature assumed by the Word of God in accord with these norms. The purpose the Word of God had in assuming human nature was the salvation and reparation of human nature. Therefore Christ had to be of such excellence in His human nature that He could fittingly be the author of man's salvation. But the salvation of man consists in the enjoyment of God, whereby man is beatified; and so Christ must have had in His human nature a perfect enjoyment of God. For the principle in any genus must be perfect. But fruition of God has a twofold aspect: it requires the satisfaction of the will and of the intellect. The will must adhere unreservedly to God by love, the intellect must know God perfectly.

Perfect attachment of the will to God is brought about by love and by grace, whereby man is justified, according to Romans 3:24: "Being justified freely by His grace." For man is made just by union with God through love. Perfect knowledge of God is effected by the light of wisdom, which is the knowledge of divine truth. Therefore the incarnate Word of God had to be perfect in grace and in the wisdom of truth. Hence we read in John 1:14: "The Word was made flesh, and dwelt among us; and we saw His glory, the glory as it were of the only begotten of the Father, full of grace and truth."