Faith is not believing there's a God, believing about God, believing who Jesus Christ is. Sure, these things are square one, but faith is coming to Christ, broken in our sin and wanting to change, coming out of darkness and self, responding to the light and God, responding like a child to the Lord drawing us and convicting our hearts, to a place of accepting Jesus Christ not only as our Savior, but as our Lord, our Master over our lives. Faith is having the Holy Spirit in you, Who opens your eyes and ears to the things of God. Faith is loving and trusting God, seeking the things of the Lord Jesus, not your own ways, your own thoughts. Christianity is not knowing about, but KNOWING God. I doubt anybody reverted to atheism, from this place.