Objection 1. It would seem that grace is not greater in one than in another. For grace is caused in us by the Divine love, as stated above (Question 110, Article 1). Now it is written (Wisdom 6:8): "He made the little and the great and He hath equally care of all." Therefore all obtain grace from Him equally.
Objection 2. Further, whatever is the greatest possible, cannot be more or less. But grace is the greatest possible, since it joins us with our last end. Therefore there is no greater or less in it. Hence it is not greater in one than in another.
Objection 3. Further, grace is the soul's life, as stated above (110, 1, ad 2). But there is no greater or less in life. Hence, neither is there in grace.
On the contrary, It is written (Ephesians 4:7): "But to every one of us is given grace according to the measure of the giving of Christ." Now what is given in measure, is not given to all equally. Hence all have not an equal grace.
I answer that, As stated above (52, A1,2; 56, A1,2), habits can have a double magnitude: one, as regards the end or object, as when a virtue is said to be more noble through being ordained to a greater good; the other on the part of the subject, which more or less participates in the habit inhering to it.
Now as regards the first magnitude, sanctifying grace cannot be greater or less, since, of its nature, grace joins man to the Highest Good, which is God. But as regards the subject, grace can receive more or less, inasmuch as one may be more perfectly enlightened by grace than another. And a certain reason for this is on the part of him who prepares himself for grace; since he who is better prepared for grace, receives more grace. Yet it is not here that we must seek the first cause of this diversity, since man prepares himself, only inasmuch as his free-will is prepared by God. Hence the first cause of this diversity is to be sought on the part of the God, Who dispenses His gifts of grace variously, in order that the beauty and perfection of the Church may result from these various degree; even as He instituted the various conditions of things, that the universe might be perfect. Hence after the Apostle had said (Ephesians 4:7): "To every one of us is given grace according to the measure of the giving of Christ," having enumerated the various graces, he adds (Ephesians 4:12): "For the perfecting of the saints... for the edifying of the body of Christ."
Reply to Objection 1. The Divine care may be looked at in two ways: first, as regards the Divine act, which is simple and uniform; and thus His care looks equally to all, since by one simple act He administers great things and little. But, "secondly," it may be considered in those things which come to be considered by the Divine care; and thus, inequality is found, inasmuch as God by His care provides greater gifts to some, and lesser gifts for others.
Reply to Objection 2. This objection is based on the first kind of magnitude of grace; since grace cannot be greater by ordaining to a greater good, but inasmuch as it more or less ordains to a greater or less participation of the same good. For there may be diversity of intensity and remissness, both in grace and in final glory as regards the subjects' participation.
Reply to Objection 3. Natural life pertains to man's substance, and hence cannot be more or less; but man partakes of the life of grace accidentally, and hence man may possess it more or less.
Source: Summa Theologica (New Advent)