Many antichrists
Whether the antichrist is a person, a spirit at work, or each who denies the Son.
1 John 2:18-23
18Little children, these are the end times, and as you heard that the Antichrist is coming, even now many antichrists have arisen. By this we know that it is the final hour.
19They went out from us, but they didn’t belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.
20You have an anointing from the Holy One, and you all have knowledge.
21I have not written to you because you don’t know the truth, but because you know it, and because no lie is of the truth.
22Who is the liar but he who denies that Jesus is the Christ? This is the Antichrist, he who denies the Father and the Son.
23Whoever denies the Son doesn’t have the Father. He who confesses the Son has the Father also.
What each tradition says
Rabbinic / Midrashic
In later Jewish eschatology and legend, Armilus is a king who will arise at the end of time against the Messiah, bring much distress upon Israel, and be conquered by him. He can be styled the Jewish Antichrist because of his relation to the Messiah, though his origin, development, and relation to Christian legend and doctrine are involved in doubt.
Kaufmann Kohler and Louis Ginzberg, Armilus (The Jewish Encyclopedia, 1901-1906) — Article opening (the Jewish Antichrist)
Patristic
Augustine says he is about to describe and mark the antichrists, and we shall see them now. He tells each person to question his own conscience whether he is an antichrist. For in our tongue antichrist means “contrary to Christ.”
Augustine, Homilies on the First Epistle of John (NPNF ser. 1, vol. 7) — Homily III on 1 John ii. 18-27, §4
Catholic
Francisco Suárez maintains that it is of faith that Antichrist is an individual person, a signal enemy of Christ. This excludes the contention of those who explain Antichrist either as the whole collection of those who oppose Jesus Christ or as the Papacy.
Anthony Maas, The Catholic Encyclopedia, vol. 1 (New York: Robert Appleton Company, 1907), s.v. 'Antichrist' — Antichrist -- In ecclesiastical language (Suarez: an individual person)
Orthodox
Cyril of Jerusalem says that if he comes to the Jews as Christ and desires to be worshipped by them, he will make great account of the Temple. He will beguile them by making it supposed that he is the man of David’s race who will build up the Temple Solomon erected.
Cyril of Jerusalem, Catechetical Lectures (NPNF ser. 2, vol. 7) — Lecture XV, §15 (on Antichrist as a man claiming to be Christ)
Reformed / Protestant
Calvin says the phrase "even now are there many antichrists" corrects those who thought there would be one kingdom. Those who suppose the antichrist would be only one man are greatly mistaken; Paul's reference to a future defection shows it would be a certain body or kingdom.
John Calvin, Commentaries on the Catholic Epistles (Calvin Translation Society) — Comment on 1 John 2:18
Evangelical
Barnes says John understood the word "antichrist" to mean all who denied that Jesus is the Messiah or that the Messiah has come in the flesh. If they held Jesus was a deceiver, not the Christ, or denied he came with a proper human nature, they showed the spirit of antichrist.
Albert Barnes, Albert Barnes' Notes on the Bible — Notes on 1 John 2:18 (who John calls antichrist)