John is warning the children of the faith that the end was at hand.  Spoken by an apostle of the Lord with such directness that there could be no doubt. He absolutely believed it was the end of something, and since we are still here we know the end he was talking about was the utter and absolute end of Jerusalem - the Nation, the temple, the Law, the geneologies - the end of it all.  John uses the term "antichrist" to describe those who oppose the doctrine of Christ.  These antichrists who once professed Jesus, showed through their apostasy they never had known Him and were therefore never really part of the body.  Since the little children of the faith had been anointed by the Holy Spirit, they knew the truth.