Objection 1. It would seem that Christ's grace is infinite. For everything immeasurable is infinite. But the grace of Christ is immeasurable; since it is written (John 3:34): "For God doth not give the Spirit by measure to His Son ['To His Son' is lacking in the Vulgate, namely Christ." Therefore the grace of Christ is infinite.
Objection 2. Further, an infinite effect betokens an infinite power which can only spring from an infinite essence. But the effect of Christ's grace is infinite, since it extends to the salvation of the whole human race; for He is the propitiation for our sins... and for those of the whole world, as is said (1 John 2:2). Therefore the grace of Christ is infinite.
Objection 3. Further, every finite thing by addition can attain to the quantity of any other finite thing. Therefore if the grace of Christ is finite the grace of any other man could increase to such an extent as to reach to an equality with Christ's grace, against what is written (Job 28:17): "Gold nor crystal cannot equal it," as Gregory expounds it (Moral. xviii). Therefore the grace of Christ is infinite.
On the contrary, Grace is something created in the soul. But every created thing is finite, according to Wisdom 11:21: "Thou hast ordered all things in measure and number and weight." Therefore the grace of Christ is not infinite.
I answer that, As was made clear above (Question 2, Article 10), a twofold grace may be considered in Christ; the first being the grace of union, which, as was said (6, 6), is for Him to be personally united to the Son of God, which union has been bestowed gratis on the human nature; and it is clear that this grace is infinite, as the Person of God is infinite. The second is habitual grace; which may be taken in two ways: first as a being, and in this way it must be a finite being, since it is in the soul of Christ, as in a subject, and Christ's soul is a creature having a finite capacity; hence the being of grace cannot be infinite, since it cannot exceed its subject. Secondly it may be viewed in its specific nature of grace; and thus the grace of Christ can be termed infinite, since it is not limited, i.e. it has whatsoever can pertain to the nature of grace, and what pertains to the nature of grace is not bestowed on Him in a fixed measure; seeing that "according to the purpose" of God to Whom it pertains to measure grace, it is bestowed on Christ's soul as on a universal principle for bestowing grace on human nature, according to Ephesians 1:5-6, "He hath graced us in His beloved Son"; thus we might say that the light of the sun is infinite, not indeed in being, but in the nature of light, as having whatever can pertain to the nature of light.
Reply to Objection 1. When it is said that the Father "doth not give the Spirit by measure," it may be expounded of the gift which God the Father from all eternity gave the Son, viz. the Divine Nature, which is an infinite gift. Hence the comment of a certain gloss: "So that the Son may be as great as the Father is." Or again, it may be referred to the gift which is given the human nature, to be united to the Divine Person, and this also is an infinite gift. Hence a gloss says on this text: "As the Father begot a full and perfect Word, it is united thus full and perfect to human nature." Thirdly, it may be referred to habitual grace, inasmuch as the grace of Christ extends to whatever belongs to grace. Hence Augustine expounding this (Tract. xiv in Joan.) says: "The division of the gifts is a measurement. For to one indeed by the Spirit is given the word of wisdom, to another the word of knowledge." But Christ the giver does not receive by measure.
Reply to Objection 2. The grace of Christ has an infinite effect, both because of the aforesaid infinity of grace, and because of the unity [Perhaps we should read 'infinity'--Ed.] of the Divine Person, to Whom Christ's soul is united.
Reply to Objection 3. The lesser can attain by augment to the quantity of the greater, when both have the same kind of quantity. But the grace of any man is compared to the grace of Christ as a particular to a universal power; hence as the force of fire, no matter how much it increases, can never equal the sun's strength, so the grace of a man, no matter how much it increases, can never equal the grace of Christ.
Source: Summa Theologica (New Advent)