In today's time, when one says "Anti-Christ", I think most people call up an image of Damien in the movie Omen (the satanic child set against God and God's people).
However, when reading the bible, you must read it in context of the time it was written, THEN apply the teaching to the present day (instead of trying to understand it using today's context).
So just looking at the term "Anti-Christ", it doesn't carry any significantly evil meaning beyond simply being against or opposed to Jesus Christ. Just like you would say I'm "Anti-Abortion" or "Anti-Slavery" or "Anti-Evolution", you are really just saying you don't agree with a particular teaching or philosphy or practice.
So, in verse 18, John is saying that people opposed to Jesus are in the world. In verse 19 it says they "went out from us". In other words, they either were originally Christian followers who fell away from the church or they were imposters originally posing as Christians...but then later started teaching against Christ or opposed the church leaders.
In verse 22-23, John states that anyone who denies Christ is the "anti-christ". Again, this is a person who has not accepted Christ, does not have the Holy Spirit living in them, and has rejected both God and Jesus Christ. Jesus said that if you don't accept Him, you are also rejecting the Father who sent him...and the scriptures testify about Him. By rejecting Him, you are rejecting God.
In verse 24-26, John is convincing you to not stray from the original message: Christ died for your sins, so that you may have eternal life through Him with the Father. Do not be led astray by ANY anti-christ...anyone teaching against Christ and His teaching. Do not be led astray by people who would say that God isn't real or that evolution created man, etc...
In addition, the Bible states that many will be led astray. I think the only real way that so many could be led astray, is for the teachings to be subtle enough, slick enough, that some will be really confused.
It would be hard for someone like a "Damien" type figure to stand up and say "follow me- and while you're at it...take this tatoo on your forehead" and for many to agree. Instead, it will need to be someone who is decietfully aimed at luring away those who are not grounded in the teachings of Christ, who have not read what God's word specifically says, and perhaps confused between what it means to be a "basically good person" and one who is a real Christ follower. I don't believe there will be any "accidentally, fortunate Christians" in heaven.
The anti- Christ, in my limited opinion, will probably teach the theory of humanism. To love everyone equally- that no one should be persecuted for following their own god. But out of the other corner of his mouth, will label Christians as hateful and to be persecuted for their limited belief in Christ as the only way to heaven.