I would think that the parable of the Prodigal Son would be sufficient enough to prove that backsliding is possible. The son is already a son...he leaves his father and squanders his life, money, and liberty in riotous living, comes to himself realizing that in his father's service, not even as a son, he would have far better, and returns, being graciously received by his father. Some try to say that this is about salvation....I don't see it that way, and it doesn't seem to be that teaching hermeneutically. One can't be a son without being a part of the family. I believe this also teaches clearly against the "Lordship" salvation theory that so many try to espouse.