An excellent article from Matins Musings:
St. Thomas explains that faith is an act of the intellect assenting to divine truth through the movement of the will. What moves the will toward faith? Grace, of course. The gift of God’s very life moves our will toward him, and if we allow that process to develop, eventually our intellect will acquiesce to divine truth. Faith, however, like the other theological virtue of hope, are animated by charity. As the Catechism of the Catholic Church states, “Fruit of the Spirit and fullness of Law, charity keeps the commandments of God and his Christ. . .” and is “the form of the virtues” (See CCC 1823 and 1827).
But what happens if, over time, we turn away from the grace of God, or simply become lazy? Every Catholic received sanctifying grace through their baptism, and continues to receive it through the eucharist. One might assume, therefore, that as long as Catholics continue to go to mass every Sunday and receive communion, they will maintain grace in their souls moving them toward divine truth.
And they will, assuming they haven’t committed mortal sin. As every Catholic knows, mortal sin destroys sanctifying grace in the soul and separates us from the love of God. Such sin principally destroys charity, which is the root of our desire to be with God. When this happens, inevitably we move further and further away from God. Our will turns back toward the self, and divine truth begins to slip away. The only thing that can repair that fissure is confession, which restores sanctifying grace to the soul.