Objection 1. It would seem that the fulness of grace is not proper to Christ. For what is proper to anyone belongs to him alone. But to be full of grace is attributed to some others; for it was said to the Blessed Virgin (Luke 1:28): "Hail, full of grace"; and again it is written (Acts 6:8): "Stephen, full of grace and fortitude." Therefore the fulness of grace is not proper to Christ.
Objection 2. Further, what can be communicated to others through Christ does not seem to be proper to Christ. But the fulness of grace can be communicated to others through Christ, since the Apostle says (Ephesians 3:19): "That you may be filled unto all the fulness of God." Therefore the fulness of grace is not proper to Christ.
Objection 3. Further, the state of the wayfarer seems to be proportioned to the state of the comprehensor. But in the state of the comprehensor there will be a certain fulness, since "in our heavenly country with its fulness of all good, although some things are bestowed in a pre-eminent way, yet nothing is possessed singularly," as is clear from Gregory (Hom. De Cent. Ovib.; xxxiv in Ev.). Therefore in the state of the comprehensor the fulness of grace is possessed by everyone, and hence the fulness of grace is not proper to Christ. on the contrary, The fulness of grace is attributed to Christ inasmuch as He is the only-begotten of the Father, according to John 1:14: "We saw Him [Vulgate: 'His glory'] as it were... the Only-begotten of the Father, full of grace and truth." But to be the Only-begotten of the Father is proper to Christ. Therefore it is proper to Him to be full of grace and truth.
I answer that, The fulness of grace may be taken in two ways: First, on the part of grace itself, or secondly on the part of the one who has grace. Now on the part of grace itself there is said to be the fulness of grace when the limit of grace is attained, as to essence and power, inasmuch as grace is possessed in its highest possible excellence and in its greatest possible extension to all its effects. And this fulness of grace is proper to Christ. But on the part of the subject there is said to be the fulness of grace when anyone fully possesses grace according to his condition--whether as regards intensity, by reason of grace being intense in him, to the limit assigned by God, according to Ephesians 4:1: "But to every one of us is given grace according to the measure of the giving of Christ"--or "as regards power," by reason of a man having the help of grace for all that belongs to his office or state, as the Apostle says (Ephesians 3:8): "To me, the least of all the saints, is given this grace... to enlighten all men." And this fulness of grace is not proper to Christ, but is communicated to others by Christ.
Reply to Objection 1. The Blessed Virgin is said to be full of grace, not on the part of grace itself--since she had not grace in its greatest possible excellence--nor for all the effects of grace; but she is said to be full of grace in reference to herself, i.e. inasmuch as she had sufficient grace for the state to which God had chosen her, i.e. to be the mother of His Only-begotten. So, too, Stephen is said to be full of grace, since he had sufficient grace to be a fit minister and witness of God, to which office he had been called. And the same must be said of others. Of these fulnesses one is greater than another, according as one is divinely pre-ordained to a higher or lower state.
Reply to Objection 2. The Apostle is there speaking of that fulness which has reference to the subject, in comparison with what man is divinely pre-ordained to; and this is either something in common, to which all the saints are pre-ordained, or something special, which pertains to the pre-eminence of some. And in this manner a certain fulness of grace is common to all the saints, viz. to have grace enough to merit eternal life, which consists in the enjoyment of God. And this is the fulness of grace which the Apostle desires for the faithful to whom he writes.
Reply to Objection 3. These gifts which are in common in heaven, viz.: vision, possession and fruition, and the like, have certain gifts corresponding to them in this life which are also common to all the saints. Yet there are certain prerogatives of saints, both in heaven and on earth, which are not possessed by all.
Source: Summa Theologica (New Advent)