I think the antichrist is not a person, but rather an "ism" or a belief. The thing that John dealt with that was against Christ was Gnosticism. He makes mention of it elsewhere, though not by name. 1 John 2:18 - "Little children, it is the End Times: and as ye heard that antichrist cometh, even now have there arisen many antichrists; whereby we know that it is the End Times." Also: 1 John 4:3 "and every spirit that confesseth not Jesus is not of God: and this is the spirit of the antichrist, whereof ye have heard that it cometh; and now it is in the world already."