From "The Habit of Being", pg. 503

The hardest thing for the writer to indicate is the presence of the anagogical which to my mind is the only thing that can cause the personality to change. Perhaps even here it changes within what it has been made. But I doubt if anyone ever touches the limits at either end of his personality. We are not our own light.

Against Determinism...

An absence of free will in these characters [in the novel The Violent Bear It Away] would mean an absence of conflict in them, whereas they spend all their time fighting within themselves, drive against drive. Tarwater wrestles with the Lord and Rayber wins. Both examples of free will in action.

Free will has to be understood within its limits; possibly we have some hinderances to free action but not enough to be able to call the world determined. In some people (psychotics) hinderances to free action may be so strong as to preclude free will in them, but the Church teaches that God does not judge those acts that are not free, and that he does not predestine any soul to hell...

I don't think literature would be possible in a determined world. We might go through the motions but the heart would be out of it. Nobody then could "smile darkly and ignore the howls." Even if there were no Church to teach me this, writing two novels would do it. I think the more you write, the less inclined you will be to rely on theories like determinism. Mystery isn't something that is gradually evaporating. It grows along with knowledge.