The word antichrist appears in only four verses of the Bible, three in 1 John and one in 2 John. In the Authorized Version it is never preceded by the definite article.  There are mentions of “the spirit of antichrist”, “an antichrist” and “many antichrists”.

Rather than being an individual who appears at the end of time, the Bible tells us that antichrist has been present since the apostolic age. It is a spirit which promotes belief systems that counterfeit true Christian doctrine, offers counterfeit salvation or a counterfeit path to knowing God.

The beast rising from the sea in Revelation 13 is a picture of how the spirit of antichrist works. The beast is a counterfeit of Christ with crowns, titles, power, death and resurrection and demand to be worshipped.

John, who wrote to warn Christians against the blasphemous powers wielded by the empire, identifies the beast as Rome (“the city on seven hills,” c.f. Rev. 17:9). But his images reveal that the power of antichrist residing in the beast is far bigger than Rome. It lives on beyond the first century, to the present and the eschaton, wherever the nature and message of Christ are refuted in the service of fraudulent demands for absolute loyalty.
– Dictionary of Bible Imagery