Behold, how great a love the Father has bestowed on us, that we should be called children of God! For this cause the world doesn't know us, because it didn't know him.

 John is not setting before us a terrifying perfectionism; but he is demanding a life which is ever on the watch against sin, a life in which sin is not the normal accepted way but the abnormal moment of defeat. John is not saying that the man who abides in God cannot sin; but he is saying that the man who abides in God cannot continue to be a deliberate sinner. 