Concerning the volitions and free-will of our Lord Jesus Christ
And further, the divine Gospel says, The Lord came into the borders of Tyre and Sidon and entered into a house, and would have no man know it; but He could not be hid. If, then, His divine will is omnipotent, but yet, though He would, He could not be hid, surely it was as man that He would and could not, and so as man He must be endowed with volition.
And once again, the Gospel tells us that, He, having come into the place, said 'I thirst': and they gave Him some vinegar mixed with gall, and when He had tasted it He would not drink. If, then, on the one hand it was as God that He suffered thirst and when He had tasted would not drink, surely He must be subject to passion also as God, for thirst and taste are passions. But if it was not as God but altogether as man that He was thirsty, likewise as man He must be endowed with volition.
Moreover, the blessed Paul the Apostle says, He became obedient unto death, even the death of the cross. But obedience is subjection of the real will, not of the unreal will. For that which is irrational is not said to be obedient or disobedient. But the Lord having become obedient to the Father, became so not as God but as man. For as God He is not said to be obedient or disobedient. For these things are of the things that are under one's hand, as the inspired Gregorius said. Wherefore, then, Christ is endowed with volition as man.
While, however, we assert that will is natural, we hold not that it is dominated by necessity, but that it is free. For if it is rational, it must be absolutely free. For it is not only the divine and uncreated nature that is free from the bonds of necessity, but also the intellectual and created nature. And this is manifest: for God, being by nature good and being by nature the Creator and by nature God, is not all this of necessity. For who is there to introduce this necessity?
It is to be observed further, that freedom of will is used in several senses, one in connection with God, another in connection with angels, and a third in connection with men. For used in reference to God it is to be understood in a superessential manner, and in reference to angels it is to be taken in the sense that the election is concomitant with the state, and admits of the interposition of no interval of time at all: for while the angel possesses free-will by nature, he uses it without let or hindrance, having neither antipathy on the part of the body to overcome nor any assailant. Again, used in reference to men, it is to be taken in the sense that the state is considered to be anterior in time to the election. For man is free and has free-will by nature, but he has also the assault of the devil to impede him and the motion of the body: and thus through the assault and the weight of the body, election comes to be later than the state.
If, then, Adam obeyed of his own will and ate of his own will, surely in us the will is the first part to suffer. And if the will is the first to suffer, and the Word Incarnate did not assume this with the rest of our nature, it follows that we have not been freed from sin.
Moreover, if the faculty of free-will which is in nature is His work and yet He did not assume it, He either condemned His own workmanship as not good, or grudged us the comfort it brought, and so deprived us of the full benefit, and showed that He was Himself subject to passion since He was not willing or not able to work out our perfect salvation.
Moreover, one cannot speak of one compound thing made of two wills in the same way as a subsistence is a composition of two natures. Firstly because the compositions are of things in subsistence (hypotasis), not of things viewed in a different category, not in one proper to them: and secondly, because if we speak of composition of wills and energies, we will be obliged to speak of composition of the other natural properties, such as the uncreated and the created, the invisible and the visible, and so on. And what will be the name of the will that is compounded out of two wills? For the compound cannot be called by the name of the elements that make it up. For otherwise we should call that which is compounded of natures nature and not subsistence. And further, if we say that there is one compound will in Christ, we separate Him in will from the Father, for the Father's will is not compound. It remains, therefore, to say that the subsistence of Christ alone is compound and common, as in the case of the natures so also in that of the natural properties.
And we cannot, if we wish to be accurate, speak of Christ as having judgment (γνώμη) and preference. For judgment is a disposition with reference to the decision arrived at after investigation and deliberation concerning something unknown, that is to say, after counsel and decision. And after judgment comes preference, which chooses out and selects the one rather than the other. But the Lord being not mere man but also God, and knowing all things, had no need of inquiry, and investigation, and counsel, and decision, and by nature made whatever is good His own and whatever is bad foreign to Him. For thus says Isaiah the prophet, Before the child shall know to prefer the evil, he shall choose the good; because before the child knows good or evil, he refuses wickedness by choosing the good. For the word “before” proves that it is not with investigation and deliberation, as is the way with us, but as God and as subsisting in a divine manner in the flesh, that is to say, being united in subsistence to the flesh, and because of His very existence and all-embracing knowledge, that He is possessed of good in His own nature. For the virtues are natural qualities, and are implanted in all by nature and in equal measure, even if we do not all in equal measure employ our natural energies. By the transgression we were driven from the natural to the unnatural. But the Lord led us back from the unnatural into the natural. For this is what is the meaning of in our image, after our likeness. And the discipline and trouble of this life were not designed as a means for our attaining virtue which was foreign to our nature, but to enable us to cast aside the evil that was foreign and contrary to our nature: just as on laboriously removing from steel the rust which is not natural to it but acquired through neglect, we reveal the natural brightness of the steel.
Observe further that the word judgment (γνώμη) is used in many ways and in many senses. Sometimes it signifies exhortation: as when the divine apostle says, Now concerning virgins I have no commandment of the Lord; yet I give my judgment: sometimes it means counsel, as when the prophet David says, They have taken crafty counsel against Your people: sometimes it means a decree, as when we read in Daniel, Concerning whom (or, what) went this shameless decree forth? At other times it is used in the sense of belief, or opinion, or purpose, and, to put it shortly, the word judgment has twenty-eight different meanings.
Source: Exposition of the Faith (New Advent)