Many deceivers have gone out into the world: These people included the Docetists, who denied that Jesus Christ came in a real human body. John argued strongly against this false teaching in his first letter (see 1 John 4:2–3).
A false teacher is a deceiver because they mislead believers who are not careful. Such a person is also an antichrist because they turn people away from the true Christ. By using this strong apocalyptic title, antichrist, John shows how serious this false teaching is and points to the judgment that false teachers will face.