18 Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour. 19 They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.
Here John talks of false teachers, those who left the early church, who didn't really belong. He calls them antichrists, not to be confused with “The Antichrist”. They were unbelievers who posed as Christians.
Have you encountered many of them? I sure have—even in the pulpit! Looks like this sort of thing has been going on from the very start of Christianity. Anyone who acts, preaches, or lives in direct opposition to Jesus’ teachings is an anti-christ.
Interesting fact: Although the word "antichrist" (Greek antikhristos) is used only in the Epistles of John, the similar word "pseudochrist" (Greek pseudokhristos, meaning "false messiah") is used by Jesus in the gospels.