Chapter 28 [XVIII.]— A Good Will Comes from God.
Men, however, are laboring to find in our own will some good thing of our own—not given to us by God; but how it is to be found I cannot imagine. The apostle says, when speaking of men's good works, “What have you that you did not receive? Now, if you received it, why do you glory, as if you had not received it?” But, besides this, even reason itself, which may be estimated in such things by such as we are, sharply restrains every one of us in our investigations so as that we may not so defend grace as to seem to take away free will, or, on the other hand, so assert free will as to be judged ungrateful to the grace of God, in our arrogant impiety.
Source: Merits and Remission of Sin, and Infant Baptism (New Advent)