John the apostle told us that, in his day, there were already many antichrists in the world and he talks about the spirit of the antichrist (1 John 4:3). An antichrist and a deceiver is someone who denies that Christ came in the flesh (2 John 1:7). This was typical of the Gnosticism of John’s day. Gnostics typically believed that the flesh is evil and the spirit is good. Therefore the gnostic version of Christ was some sort of spectre, who was not born of the virgin Mary (cf. Matthew 1:20, 23). The early church wrote heavily against these ideas and many Christians think that the writers of the early church won against those groups but, my opinion is that, sadly, their ideas just infiltrated, step by step, under Ambrosiaster, Origen, Ambrose, Jerome, Augustine,… You can even hear it today in churches, in sayings like: “We are born in sinful nature/flesh, we cannot help but sin every day because we inherit sin from Adam but Jesus doesn’t have to sin because He has different flesh.” It is a spirit of lawlessness which redefines sin and makes it an inevitable thing instead of an act of the will (see 1 John 3:4 and following verses). By redefining core doctrines (cf. Jude 1:3), it gives an “excuse” to religious sinners (cf. Matthew 7:17-23). I believe these things become clear as you study 1 & 2 John and the writings of the early Christians.
I think well-meaning Catholics and Orthodox believe that the antichrist was defeated, when Gnosticism supposedly was defeated and the Protestants think he is some form of future leader, while the devil tries to corrupt the church from the inside out.
picture source: Ernesto Carrasco (musingsofernie).
“Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.” - 1 John 4:2-3 (KJV)
(Article adapted from a message I wrote to a church chat group.)