Objection 1. It would seem there was no habitual grace in the soul assumed by the Word. For grace is a certain partaking of the Godhead by the rational creature, according to 2 Peter 1:4: "By Whom He hath given us most great and precious promises, that by these you may be made partakers of the Divine Nature." Now Christ is God not by participation, but in truth. Therefore there was no habitual grace in Him.
Objection 2. Further, grace is necessary to man, that he may operate well, according to 1 Corinthians 15:10: "I have labored more abundantly than all they; yet not I, but the grace of God with me"; and in order that he may reach eternal life, according to Romans 6:23: "The grace of God (is) life everlasting." Now the inheritance of everlasting life was due to Christ by the mere fact of His being the natural Son of God; and by the fact of His being the Word, by Whom all things were made, He had the power of doing all things well. Therefore His human nature needed no further grace beyond union with the Word.
Objection 3. Further, what operates as an instrument does not need a habit for its own operations, since habits are rooted in the principal agent. Now the human nature in Christ was "as the instrument of the Godhead," as Damascene says (De Fide Orth. iii, 15). Therefore there was no need of habitual grace in Christ.
On the contrary, It is written (Isaiah 11:2): "The Spirit of the Lord shall rest upon Him"--which (Spirit), indeed, is said to be in man by habitual grace, as was said above (I, 8, 3; I, 43, 3,6). Therefore there was habitual grace in Christ.
I answer that, It is necessary to suppose habitual grace in Christ for three reasons. First, on account of the union of His soul with the Word of God. For the nearer any recipient is to an inflowing cause, the more does it partake of its influence. Now the influx of grace is from God, according to Psalm 83:12: "The Lord will give grace and glory." And hence it was most fitting that His soul should receive the influx of Divine grace. Secondly, on account of the dignity of this soul, whose operations were to attain so closely to God by knowledge and love, to which it is necessary for human nature to be raised by grace. Thirdly, on account of the relation of Christ to the human race. For Christ, as man, is the "Mediator of God and men," as is written, 1 Timothy 2:5; and hence it behooved Him to have grace which would overflow upon others, according to John 1:16: "And of His fulness we have all received, and grace for grace."
Reply to Objection 1. Christ is the true God in Divine Person and Nature. Yet because together with unity of person there remains distinction of natures, as stated above (2, 1,2), the soul of Christ. is not essentially Divine. Hence it behooves it to be Divine by participation, which is by grace.
Reply to Objection 2. To Christ, inasmuch as He is the natural Son of God, is due an eternal inheritance, which is the uncreated beatitude through the uncreated act of knowledge and love of God, i.e. the same whereby the Father knows and loves Himself. Now the soul was not capable of this act, on account of the difference of natures. Hence it behooved it to attain to God by a created act of fruition which could not be without grace. Likewise, inasmuch as He was the Word of God, He had the power of doing all things well by the Divine operation. And because it is necessary to admit a human operation, distinct from the Divine operation, as will be shown (19, 1), it was necessary for Him to have habitual grace, whereby this operation might be perfect in Him.
Reply to Objection 3. The humanity of Christ is the instrument of the Godhead--not, indeed, an inanimate instrument, which nowise acts, but is merely acted upon; but an instrument animated by a rational soul, which is so acted upon as to act. And hence the nature of the action demanded that he should have habitual grace.
Source: Summa Theologica (New Advent)