1 John 2:18-19

My dear children, these are the last days. You have heard that the enemy of Christ is coming, and now many enemies of Christ are already here. This is how we know that these are the last days. These enemies of Christ were in our fellowship, but they left us. They never really belonged to us; if they had been a part of us, they would have stayed with us. But they left, and this shows that none of them really belonged to us.