In order to leave, you have to first be in it. You can’t leave a place you were never in. You can’t leave Cleveland if you were never in Cleveland.
But you can claim to be in faith without really being in the faith. That is John’s entire point. God knew from the moment of creation what your and my ultimate fate would be. Why would God save someone he knows he will have to unsave later, maybe back and forth several times? That would be an imperfect God and I don’t believe in an imperfect God.