I answer: there are two ways of fulfilling the commandments of the law. In the first place, one may actually do what the law commands, by performing acts of justice or fortitude, for example, or other acts of virtue. Man could fulfill all the commandments of the law in this way when he was in the state of pure nature, since he would not otherwise have been able to avoid sin, which is nothing other than transgression of the divine commandments. But a man in the state of corrupt nature cannot fulfill all the divine commandments without healing grace. In the second place, the law may be fulfilled not only in respect of what it commands, but also in respect of the manner of action. It is fulfilled when actions are inspired by charity [love]. A man cannot fulfill the law in this way without grace, whether in the state of pure nature or in the state of corrupt nature.
Amen and amen! This is almost exactly what C. S. Lewis says in his essay "Three Kinds of Men," which I discovered and commented on here. What a vital truth, one shot through the entire Bible, OT and New.
How often I fall into externalized, backpatting, self-congratulatory, misery-inducing, joy-depleting obedience which is not obedience at all but Pharisaism. Maybe you do too. Let's remember Thomas' words.