Objection 1. It would seem that the grace of Christ could increase. For to every finite thing addition can be made. But the grace of Christ was finite. Therefore it could increase.
Objection 2. Further, it is by Divine power that grace is increased, according to 2 Corinthians 9:8: "And God is able to make all grace abound in you." But the Divine power, being infinite, is confined by no limits. Therefore it seems that the grace of Christ could have been greater.
Objection 3. Further, it is written (Luke 2:52) that the child "Jesus advanced in wisdom and age and grace with God and men." Therefore the grace of Christ could increase.
On the contrary, It is written (John 1:14): "We saw Him [Vulgate: 'His glory'] as it were... the Only-begotten of the Father, full of grace and truth." But nothing can be or can be thought greater than that anyone should be the Only-begotten of the Father. Therefore no greater grace can be or can be thought than that of which Christ was full.
I answer that, For a form to be incapable of increase happens in two ways: First on the part of the subject; secondly, on the part of the form itself. On the part of the subject, indeed, when the subject reaches the utmost limit wherein it partakes of this form, after its own manner, e.g. if we say that air cannot increase in heat, when it has reached the utmost limit of heat which can exist in the nature of air, although there may be greater heat in actual existence, viz. the heat of fire. But on the part of the form, the possibility of increase is excluded when a subject reaches the utmost perfection which this form can have by nature, e.g. if we say the heat of fire cannot be increased because there cannot be a more perfect grade of heat than that to which fire attains. Now the proper measure of grace, like that of other forms, is determined by the Divine wisdom, according to Wisdom 11:21: "Thou hast ordered all things in number, weight and measure." And it is with reference to its end that a measure is set to every form. as there is no greater gravity than that of the earth, because there is no lower place than that of the earth. Now the end of grace is the union of the rational creature with God. But there can neither be nor be thought a greater union of the rational creature with God than that which is in the Person. And hence the grace of Christ reached the highest measure of grace. Hence it is clear that the grace of Christ cannot be increased on the part of grace. But neither can it be increased on the part of the subject, since Christ as man was a true and full comprehensor from the first instant of His conception. Hence there could have been no increase of grace in Him, as there could be none in the rest of the blessed, whose grace could not increase, seeing that they have reached their last end. But as regards men who are wholly wayfarers, their grace can be increased not merely on the part of the form, since they have not attained the highest degree of grace, but also on the part of the subject, since they have not yet attained their end.
Reply to Objection 1. If we speak of mathematical quantity, addition can be made to any finite quantity, since there is nothing on the part of finite quantity which is repugnant to addition. But if we speak of natural quantity, there may be repugnance on the part of the form to which a determined quantity is due, even as other accidents are determined. Hence the Philosopher says (De Anima ii, 41) that "there is naturally a term of all things, and a fixed limit of magnitude and increase." And hence to the quantity of the whole there can be no addition. And still more must we suppose a term in the forms themselves, beyond which they may not go. Hence it is not necessary that addition should be capable of being made to Christ's grace, although it is finite in its essence.
Reply to Objection 2. Although the Divine power can make something greater and better than the habitual grace of Christ, yet it could not make it to be ordained to anything greater than the personal union with the Only-begotten Son of the Father; and to this union, by the purpose of the Divine wisdom, the measure of grace is sufficient.
Reply to Objection 3. Anyone may increase in wisdom and grace in two ways. First inasmuch as the very habits of wisdom and grace are increased; and in this way Christ did not increase. Secondly, as regards the effects, i.e. inasmuch as they do wiser and greater works; and in this way Christ increased in wisdom and grace even as in age, since in the course of time He did more perfect works, to prove Himself true man, both in the things of God, and in the things of man.
Source: Summa Theologica (New Advent)