John says this: Who is the liar, if not the one who denies that Jesus is the Christ? That person is an antichrist, who denies the Father and the Son (1 John 2.22).
The apostles emphasizes that a rejection of Jesus as Christ is a rejection of God the Father, as well, which echoes a sentiment which Jesus himself expressed: No one goes to the Father except by me, and If you've known me, you know my Father as well, and Whoever sees me, sees the Father (John 14.6-7, 9).
There is an essential unity between Jesus and the Father, then, such that the denial of one entails the denial of the other. The Father is totally invested in mediating his communion with and salvation of humanity through his Son Jesus Christ; the Son has entirely dedicated his life to the service of and obedience to the Father for the sake of humanity. Their identities are intricately intertwined and interrelated, to the point that they can not be approached or dealt with in isolation from one another.
This is a critical piece of information for the debate regarding the salutary status of other religions: can a person who is not a Christian but a committed member of a different religion be saved?
Insofar as God the Father and Christ his Son are so closely related, there certainly can't be the pretension that we may approach by other means than through Jesus Christ. The problem then is raised: does this require explicit faith in Christ, or can Christ be doing some salvific work unknowingly in the life of a person who is not an explicit Christian?
These are interesting questions, ones which I am not intending to answer here. I wish only to emphasize John's point here that Jesus and the Father are one. If we look Jesus in the face and deny him, we have denied God the Father as well -- something I'm sure his fellow Jewish nationals did not take lightly, though Christ told them as much as well.