The crazy thing about 1John 2:18,19 is that John himself was creating false doctrine by his statement.
Jesus said that if a sheep leaves the flock he should be searched out and restored.
The apostle John was a grumpy, disgruntled Christian who thought Jesus would return in his life time.
John probably believed the idea that he would be alive when Jesus returned (see John 21:20-23) "If it is my will for him to remain until I come of what concern is that to you?...In consequence, this saying went out among the brothers, that that disciple (John) would not die."
Old John wanted to believe that Jesus was coming soon because he was approaching death. So he switched the meaning of Antichrist from the traditional belief that it is a wicked world ruler to being any person that quit associating. This is a good example of switching the meaning by creating a "class" of people out of a single actor. There was one Christ and there can only be one Anti-Christ.
Jehovah's Witnesses have been twisting things just like the early christians.