They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 John says that he who denies the Son has not the Father either. What lies behind that saying is this. The false teachers pleaded, "It may be that we have different ideas from yours about Jesus; but you and we do believe the same things about God." John's answer is that that is an impossible position; no man can deny the Son and still have the Father. How does he arrive at this view? 