an opponent and counterfeit of Christ, identified by denying that Jesus is the Christ come in the flesh
This word is found only in John's letters, where it always points to the same thing: someone set against Christ. The prefix carries two notes at once. It can mean "against," naming an enemy who fights what Christ stands for, and it can mean "in place of," naming a rival who tries to take Christ's place. John seems to hold both together, because what makes a false teacher so dangerous is that he competes with the true confession from the inside while opposing it at heart.
The test John applies is not behavior but belief. The mark of an antichrist is that he denies that Jesus is the Christ, refusing to confess that Jesus Christ has truly come in human flesh. So the word is tied to a specific failure of faith about who Jesus is, not to a generic image of evil.
The two senses below are one idea seen from two angles. His readers had heard that a single antichrist was still to come, an expected adversary of the last days. John tells them that many antichrists have already appeared, the teachers who broke away from his community. He uses the loaded title to say the last hour has arrived: the figure they were waiting for is already at work in these present deniers.