Levin's soul is affected by his conversation with Theodore. As he ponders it more deeply, he realizes that he has discovered nothing that he didn't know already. Rather than living only for himself, he has been living life for some greater cause that he was unable to name—he has been living for God all along.
Levin thinks about personal needs being subordinated to the service of truth. He realizes that the life of the spirit is the only thing worth living for. If you try to do good to accomplish something, it is no longer good. You have to do good for its own sake, and that is service to God.
This epiphany could never come from reason, because rational thinking is all about cause and effect. Good is outside rationality, and has to come from whatever it is that we call love or God. This faith is beyond the rationale even of the Church. He replaces the teachings of the Church itself with a belief in the importance of serving good instead of one's needs.