Scripture describes for us the sin of being antichrist, the Greek word being antichristos. There are four uses of the word in 1 John, and one in 2 John, and that’s it for the Bible. Surprising to many, the antichrist is not found in the book of Revelation at all. The recipients of John’s letter had heard that the antichrist was going to come, and indeed, John says, many antichrists had already come (1 Jn. 2:18). The antichrist is defined as one who denies the Father and the Son (1 Jn. 2:22). And the spirit of antichrist is a refusal to confess that Jesus Christ had come in the flesh (1 Jn. 4:3). The same thing is said again in 2 Jn. 1:7. The spirit of deception and antichrist is a rejection of Jesus come in the flesh.
So what is the sin of being an antichrist. Through long-standing misunderstandings about eschatology, the definition of this sin has gotten almost completely distorted. A common understanding is to see The Antichrist and The Beast as the same character out of poorly written end times novels. But this is not the case at all. In Scripture, a beast is a civil ruler, persecuting the Church. An antichrist is a false teacher from within, one infected with all the latest ideational leprosy. For a beast, think Stalin, Hitler, Nero. For an antichrist, think of a mild, soften-spoken Anglican bishop — one who denies that Jesus was God enfleshed.