John writes to his audience about 'antichrist' in 1 John 2. Actually, he makes the point that there are multiple antichrists, but they have something in common: they deny that Jesus is the messiah, the son of God.
These persons were probably (to my mind, anyway) Jews who had denied Jesus as the messiah in the persecutions and difficulties which affected the Jewish synagogues after the destruction of Jerusalem and the temple. After the destruction of the temple, the Pharisees took a very hardline stance against the "Nazarenes," the followers of Jesus. Because of this, many Jews had to choose between the fellowship of the synagogue or else following Christ in rejection by their communities. From what I have read, the majority may have chosen the synagogue and the social stability it offered over Christ.
So John writes:
Who is the liar but the one who denies that Jesus is the Christ?This is the antichrist, the one who denies the Father and the Son. No one who denies the Son has the Father; everyone who confesses the Son has the Father also (1 John 2.22-3).
He has to write to his audience to assure them that, though they are perhaps refused and excommunicated by the local synagogue, they are nevertheless enjoying fellowship with God. Why is is this? Because they have accepted his Son, Jesus the true Christ.
And in the middle of his exhortation and consoloation, John actually espouses quite a strict exclusivism: the relationship between humanity and God is mediated through his Son, Jesus Christ (cf. 1 Tim 2.5), so that whoever rejects the Son whom the Father had sent, such a person also rejects the Father. Apart from the worship and the acceptance of Christ, there is no fellowship with the Father.
This is quite a strict stance to take, especially in light of contemporary ecumenical relations and the impulse to interfaith dialog. John goes so far as to call the Jewish deniers of Jesus anti-Christs, who have neither the Son, nor the Father. Their relationship and fellowship with God has been utterly severed because of their rejection of God's Son whom he sent to them for their salvation.