While we don’t really get an explicit “The Antichrist is a future enemy of the people of God that will show up at some point” anywhere in the Bible. From very early on Christians saw, through many passages, what appeared to be describing such a figure; the precise texts Christians used varied a little, but the main ones were the Beast in the Revelation, the Man of Sin in 2 Thessalonians, and where St. John in his 1st Epistle says “You have heard Antichrist is coming”.
This led a handful of prominent early Christian theologians, especially St. Irenaeus and St. Hippolytus, to go into some detail. Hippolytus, especially, wrote one of the earliest treatises on the subject “On Christ and Antichrist”.
In time this led to greater speculations–who is the Antichrist? And the game of pin the tail on the donkey has been going on for centuries. Muhammad, Popes, politicians, princes, emperors, to modern presidents and celebrities have all been imagined as the Antichrist.
It’s been an excessively long time since I read it, and I have been unable to find the source which makes me wonder if I dreamed it up somehow–but I recall reading a statement from the great St. Augustine saying we should be less concerned with trying to figure out who the Antichrist might be; and more concerned that we don’t, ourselves, become antichrists–that is the real danger for the Christian isn’t necessarily an eschatological bogeyman; but a heart that resists following Jesus.
If you were to ask me what “antichrist” means in the most explicitly biblical sense (i.e., where the text explicitly uses this term) I think St. John is pretty clearly talking about certain heretics. These heretics were going around teaching dangerous doctrines, specifically a form of proto-Gnosticism.
Ancient tradition links St. John to an early heretic by the name of Cerinthus gets presented as a kind of archnemesis to John. Cerinthus lends his name to a doctrine known as Cerinthianism; Cerinthus taught a form of Docetism, making a distinction between the man Jesus and the divine Christ. Cerinthus taught that at Jesus’ baptism “Christ” descended (one imagines he conflates “Christ” here with the Holy Spirit who descended at Jesus’ baptism in the form of a dove) and effectively possessed, or took full control and agency. Then at the crucifixion “Christ” departs, leaving the man Jesus hanging on the cross lonely and afraid.
This explains some of the statements St. John makes in his epistles about those who deny Jesus is the Christ, who deny the Son, that Jesus Christ became flesh, etc. These are condemnatory statements directed at Docetic beliefs: that Jesus Christ only seemed (from the Greek verb dokeo, meaning “to seem”) to be human. Later Docetic works like the Acts of John are super-explicit in their rejection of Christ’s humanity, describing Jesus taking on various forms at will, or not leaving footprints in the sand, or where sometimes when trying to touch Jesus he would be solid while other times hands went right through him like he wasn’t even there.
I think what St. John is calling antichrist are these teachings and teachers, these are of “the spirit of antichrist”; he’s saying these things are against Christ, set against Him, opposite of Him, preaching such rank and grotesque falsehood that they aren’t just wrong, they are ANTI-Christ.
That said, the phrase “you have heard antichrist is coming” is still in the text. Though perhaps relevant is whether St. John wrote ὅτι ὁ ἀντίχριστος “that the antichrist” or ὅτι ἀντίχριστος “that antichrist”. The inclusion of exclusion of the definite article is relevant; while the exclusion doesn’t necessarily rule out the meaning of a definitive capital-A Antichrist; it does leave room for a more general sense, “an antichrist”? Perhaps, or simply meaning that what is coming is that (not necessarily a person) antichrist (that which opposes Christ) is coming, because “it is the last hour”.
My opinion: I don’t believe there is enough biblical material to say, at a dogmatic level, that there is or will be a capital-A Antichrist. I don’t reject it, but I also don’t argue for it as a matter of dogma.