Here's what Scripture (specifically John) says about the "antichrist":
1 John 2:18
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.
1 John 2:22
Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist—he denies the Father and the Son.
1 John 4:3
but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
2 John 1:7
Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist.
Here's what we know from these passages:
The "antichrist" was coming, but in fact, many "antichrists" had come.
So, who is it? -->
1) The man who denies that Jesus is the Christ.
2) Denies the Father and the Son.
3) Does not acknowledge Jesus.
4) Does not acknowledge Jesus Christ as coming in the flesh.
That's pretty much what we know. Actually, much of this may refer to false teachings by Gnostics, Docetists, and other groups at the time who were saying that Jesus, as God in the flesh, didn't really physically die on the cross.