First we shall deal with the question of the fullness of grace in Christ. In this matter we should observe that the term, grace, may be understood in two senses. According to one usage, it means to be pleasing (gratum): we say that someone is in the good graces of another because he is pleasing to him. In another sense, it means that something is given gratis: a person is said to grant a grace to another when he confers gratis a benefit on that other.
These two meanings of grace are not wholly unconnected. A thing is given gratis to another because he to whom it is given is pleasing (gratus) to the giver, either simply or in some respect. The recipient is simply pleasing to the giver, when he is pleasing to such an extent that the giver associates him with himself in some way. For those whom we hold dear (quos gratos habemus) we attract to ourselves as far as we can, according to the quantity and degree in which they are dear to us. But the recipient is pleasing to the giver only in some respect when he is pleasing to the extent that he receives something from him, although not to the extent that he is taken into association by the donor. Clearly, therefore, everyone who has favor (qui habet gratiam) with another, has something given to him gratis; but not everyone who has something given to him gratis is pleasing (gratus) to the donor. Hence we ordinarily distinguish between two kinds of grace: one, namely, which is only given gratis, and the other which, in addition, makes pleasing (gratum facit). A thing is said to be given gratis if it is in no way due.
A thing may be due in two ways, either according to nature or according to operation. According to nature, whatever the natural order of a thing requires, is due to it; thus the possession of reason and hands and feet is due to man. According to operation, a thing is due in the way that a recompense is due to a worker. Therefore those gifts are given gratis by God to men, which exceed the order of nature and are not acquired by merits; although even gifts that are conferred by God because of merits sometimes retain the name and character of grace, because the principle of meriting comes from grace, and also because rewards are given over and above what human merits require, as we learn from Romans 6:23: “The grace of God, life everlasting.”
Among such gifts, some exceed the capacity of human nature and are not given for merits. However, the fact that a man has these gifts does not prove that he is thereby rendered pleasing to God. Examples are the gifts of prophecy, of working miracles, of knowledge, of teaching, or any other such gifts divinely conferred. By these and like gifts man is not united to God, except, perhaps, by a certain similarity, so far as he shares to some extent in His goodness; but everything is assimilated to God in this way. Yet some gifts do render man pleasing to God, and join man to Him. Such gifts are called graces, not only because they are given gratis, but also because they make man pleasing to God.
The union of man with God is twofold. One way is by affection, and this is brought about by charity, which in a certain sense makes man one with God in affection, as is said in 1 Corinthians 6:17: “He who is joined to the Lord is one spirit.” Through this virtue God dwells in man, according to John 14:23: “If anyone love Me, he will keep My word, and My Father will love him, and We will come to him and will make Our abode with him.” It also causes man to be in God, according to I John 4: 16: “He who abides in charity abides in God, and God in him.” By receiving this gratuitous gift, therefore, man is made pleasing to God, and he is brought so far that by the love of charity he becomes one spirit with God: he is in God and God is in him. Hence the Apostle teaches, in 1 Corinthians 13:1-3, that without charity the other gifts do not profit men: they cannot make men pleasing to God unless charity is present.
This grace is common to all the saints. And so the man Christ, when asking for this grace for His disciples in prayer, begs: “That they may be one,” namely, by the bond of love, as We also are one” (John 17:22).
There is another conjunction of man with God that is brought about, not only by affection or inhabitation, but also by the unity of hypostasis or person, so that one and the same hypostasis or person is both God and man. And this conjunction of man with God is proper to Jesus Christ. We have already spoken at length of this union. In truth, this is a singular grace of the man Christ, that He is united to God in unity of person. Clearly the grace was given gratis, for it exceeds the capacity of nature, and besides, there were no merits to precede this gift. But it also makes Him supremely pleasing to God, so that the Father says of Him in a unique sense: “This is My beloved Son, in whom I am well pleased” (Matt. 3: 17; 17: 5),
The difference between these two graces seems to be as follows. The grace whereby man is united to God by affection exists in the soul as something habitual. For that union is accomplished through an act of love, and perfect acts issue from habit; consequently some habitual grace is infused into the soul to produce that eminently perfect habit whereby the soul is united to God by love. On the other hand, personal or hypostatic being is constituted, not by any habit, but by the natures to which the hypostases or persons pertain. Therefore the union of human nature with God in unity of person is brought about, not by some habitual grace, but by the conjunction of the natures themselves in one person.
The closer any creature draws to God, the more it shares in His goodness, and the more abundantly it is filled with gifts infused by Him. Thus he who comes closer to a fire, shares to a greater extent in its heat. But there can be no way, nor can any be imagined, by which a creature more closely adheres to God, than by being united to Him in unity of person. Therefore, in consequence of the very union of His human nature with God in unity of person, Christ’s soul was filled with habitual gifts of graces beyond all other souls. And so habitual grace in Christ is not a disposition for union, but is rather an effect of union.
This appears clearly in the very way of speaking used by the Evangelist when he says, in words previously quoted: “We saw [Him as it were] the only begotten of the Father, full of grace and truth” (John 1:14). The man Christ is, indeed, the only-begotten of the Father, inasmuch as the Word was made flesh. The very fact that the Word was made flesh entailed the consequence that He was full of grace and truth. But among things that are filled with any goodness or perfection, the one from which goodness or perfection flows out upon other things is found to be filled to greater repletion; for example, what can shed light on other objects, shines more brilliantly than they. Therefore, since the man Christ possessed supreme fullness of grace, as being the only-begotten of the Father, grace overflowed from Him to others, so that the Son of God, made man, might make men gods and sons of God, according to the Apostle’s words in Galatians 4:4 ff.: “God sent His Son, made of a woman, made under the law, that He might redeem them who were under the law: that we might receive the adoption of sons.”
Because of the fact that grace and truth come to others from Christ, it is fitting that He should be the head of the Church. Sensation and movement are, in a way, conveyed from the head to the other members that are conformed to the head in nature. In like manner grace and truth are conveyed from Christ to other men. Hence we are told in Ephesians 1:22 ff., that God “made Him head over all the Church, which is His body.” Christ is not the head of men alone; He can also be called the head of the angels, at least with respect to His excellence and influence, if not with respect to conformity of nature in the same species. This is why, before the words just quoted, the Apostle says that God set Him, namely Christ, “on His right hand in the heavenly places, above all principality and power and virtue and dominion.”
In accord with this doctrine, a threefold grace is usually pointed out in Christ. The first is the grace of union, whereby the human nature, with no merits preceding, received the gift of being united in person to the Son of God. The second is the singular grace whereby the soul of Christ was filled with grace and truth beyond all other souls. The third is the grace of being head, in virtue of which grace flows from Him to others. The Evangelist presents these three kinds of grace in due order (John 1:14, 16). Regarding the grace of union he says: “The Word was made flesh.” Regarding Christ’s singular grace he says: “We saw [Him as it were] the only begotten of the Father, full of grace and truth.” Regarding the grace of head he adds: “And of His fullness we all have received.”
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Fullness of Christ’s Grace, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).