Read the story of Charles Templeton if you dare. This is the story of a man who was saved and preached the gospel for years and lost his faith by believing evolution rather then the bible. He turned from God and never came back...it can happen.
You need to read what Ariel wrote about Templeton and his last days on this earth. He was calling after Jesus and saying he missed him. We can get screwed up in our minds and still have the heart intact - for it is with the heart man believes.
Perhaps Ariel can give us the quote again? I think it was Ariel.