The antichrists were claiming faith in God while denying and opposing the Son. To do, John says, is impossible. Because Jesus is God's Son and the Messiah, anyone who denies the Son doesn't have the Father either. And, anyone who confesses the Son also has the Father. Many cultists today call themselves Christians, but they deny the deity of Jesus Christ.
True believers, who had been begotten of God had received the Holy Spirit to help the discern truth and reject falsehood. They didn't need another teacher. Persevering in the relationship with Christ ensures that believers will remain faithful.
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.