Antichrist is a multi-million dollar enterprise. There are countless books predicting the rise of the antichrist, showing (if not who specifically) exactly when and where and under what circumstances the person will appear.
According to John, however, there are lots of antichrists and they have always been around. Here is what he says:
Who is the liar? Isn’t it the person who denies that Jesus is the Christ? This person is the antichrist: the one who denies the Father and the Son. Everyone who denies the Son does not have the Father, but the one who confesses the Son has the Father also.
Anyone who denies God is antichrist. Specifically, anyone who denies the Son is God does not have the Father, and therefore is antichrist. This flows right out of the theology in the Gospel of John, where Jesus tells his disciples that if they have seen him they have seen the Father, Colossians 1:15-20, and Hebrews 1:1-4.
This is why, contrary to some opinions, Islam cannot be counted in the same family tree as Christianity. The Islamic faith does have a place for Jesus, but their Jesus is not the Jesus of the New Testament. They deny that Jesus is God the Son, the second person of the Trinity–Light from Light, true God from true God, of one substance with the Father. Their Jesus is a prophet who was mighty in word and deed, but paltry and weak compared to the Christian truth about him.
This also cuts to the heart of much liberal (Christian and otherwise) scholarship concerning Jesus, which feeds into the image of a holy person (perhaps a prophet) who was just more connected to God than other people. This is antichrist, since it denies God the Son–and therefore those who hold to such interpretations do not even have the Father, according to John.
For an understanding of Jesus to be Christian, it has to know Jesus as God in the flesh. Anything less than this understanding is antichrist. And if Jesus is God, then those who would follow him have a duty and responsibility to obey him. This is not always easy. Just look at this write-up of one of his sermons.