"If that commandment is observed out of fear of punishment, not out of the love of righteousness, it is observed in the manner of a slave, not in the manner of someone free, and for that reason it is not observed. For there is lacking the good fruit that springs up from the root of love. But if faith that works through love is present (Gal 5:6), one begins to find delight in the law of God in the interior human being. This delight is a gift, not of the letter, but of the Spirit. . . ."
"The Lord is the Spirit, but where the Spirit of the Lord is, there is freedom (2 Cor 3:17). This is the Spirit of God by whose gift we are justified; by his gift there comes to be in us a delight in not sinning so that we have freedom. So too, without this Spirit we find delight in sinning so that we are enslaved.
"Are we then doing away with free choice through grace? Heaven forbid! Rather, we make free choice stronger. . . . through faith we obtain grace to struggle against sin; through grace the soul is healed from the wound of sin; through the good health of the soul we have freedom of choice; through free choice we have the love of righteousness; through the love of righteousness we fulfill the law. The law is not done away with, but strengthened by faith, because faith obtains the grace by which we fulfill the law. In the same way, free choice is not done away with by grace, but strengthened, because grace heals the will by which we freely love righteousness."
--“The Spirit and the Letter,” in The Works of Saint Augustine, Vol. 23