Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son. Who is a liar but he that denieth that Jesus is the Christ? - Here we see some of the false doctrines which were then propagated in the world. There were certain persons who, while they acknowledged Jesus to be a Divine teacher, denied him to be the Christ, i.e. the Messiah.
He is antichrist, that denieth the Father and the Son, - He is antichrist who denies the supernatural and miraculous birth of Jesus Christ, who denies Jesus to be the Son of God, and who denies God to be the Father of the Lord Jesus; thus he denies the Father and the Son. The Jews in general, and the Gnostics in particular, denied the miraculous conception of Jesus; with both he was accounted no more than a common man, the son of Joseph and Mary. But the Gnostics held that a Divine person, Aeon, or angelical being, dwelt in him; but all things else relative to his miraculous generation and Divinity they rejected. These were antichrists, who denied Jesus to be the Christ.