1 John deals with the spirit of antichrist, which was even in existence in the first century. 2 Thessalonians deals with the man who becomes THE Antichrist, the one-world leader opposed to God who sets himself up as God and institutes the mark of the beast. This is the man that the book of Revelation refers to as the beast. 2 Thessalonians (which refers to him as the son of perdition, the same title Judas had when possessed by Satan) and Revelation 12 also teach/imply that this man will become possessed by Satan at the halfway point of the tribulation (3 1/2 year point), when Satan no longer has access to Heaven to accuse the brethren before God, so he goes on a rampage against all the Jews and those that got saved during the tribulation.