Objection 1. It would seem that in Christ there was not the fulness of grace. For the virtues flow from grace, as was said above (I-II, 110, 4). But in Christ there were not all the virtues; for there was neither faith nor hope in Him, as was shown above (3,4). Therefore in Christ there was not the fulness of grace.
Objection 2. Further, as is plain from what was said above (I-II, 111, 2), grace is divided into operating and cooperating. Now operating grace signifies that whereby the ungodly is justified, which has no place in Christ, Who never lay under any sin. Therefore in Christ there was not the fulness of grace.
Objection 3. Further, it is written (James 1:17): "Every best gift and every perfect gift is from above, coming down from the Father of lights." But what comes thus is possessed partially, and not fully. Therefore no creature, not even the soul of Christ, can have the fulness of the gifts of grace.
On the contrary, It is written (John 1:14): "We saw Him [Vulgate: 'His glory'] full of grace and truth."
I answer that, To have fully is to have wholly and perfectly. Now totality and perfection can be taken in two ways: First as regards their "intensive" quantity; for instance, I may say that some man has whiteness fully, because he has as much of it as can naturally be in him; secondly, "as regards power"; for instance, if anyone be said to have life fully, inasmuch as he has it in all the effects or works of life; and thus man has life fully, but senseless animals or plants have not. Now in both these ways Christ has the fulness of grace. First, since He has grace in its highest degree, in the most perfect way it can be had. And this appears, first, from the nearness of Christ's soul to the cause of grace. For it was said above (Article 1) that the nearer a recipient is to the inflowing cause, the more it receives. And hence the soul of Christ, which is more closely united to God than all other rational creatures, receives the greatest outpouring of His grace. Secondly, in His relation to the effect. For the soul of Christ so received grace, that, in a manner, it is poured out from it upon others. And hence it behooved Him to have the greatest grace; as fire which is the cause of heat in other hot things, is of all things the hottest.
Likewise, as regards the "virtue" of grace, He had grace fully, since He had it for all the operations and effects of grace; and this, because grace was bestowed on Him, as upon a universal principle in the genus of such as have grace. Now the virtue of the first principle of a genus universally extends itself to all the effects of that genus; thus the force of the sun, which is the universal cause of generation, as Dionysius says (Div. Nom. i), extends to all things that come under generation. Hence the second fulness of grace is seen in Christ inasmuch as His grace extends to all the effects of grace, which are the virtues, gifts, and the like.
Reply to Objection 1. Faith and hope signify effects of grace with certain defects on the part of the recipient of grace, inasmuch as faith is of the unseen, and hope of what is not yet possessed. Hence it was not necessary that in Christ, Who is the author of grace, there should be any defects such as faith and hope imply; but whatever perfection is in faith and hope was in Christ most perfectly; as in fire there are not all the modes of heat which are defective by the subject's defect, but whatever belongs to the perfection of heat.
Reply to Objection 2. It pertains essentially to operating grace to justify; but that it makes the ungodly to be just is accidental to it on the part of the subject, in which sin is found. Therefore the soul of Christ was justified by operating grace, inasmuch as it was rendered just and holy by it from the beginning of His conception; not that it was until then sinful, or even not just.
Reply to Objection 3. The fulness of grace is attributed to the soul of Christ according to the capacity of the creature and not by comparison with the infinite fulness of the Divine goodness.
Source: Summa Theologica (New Advent)