It is hard to outline the meaning of AntiChrist given the usage in today's culture. However, when John was writing to his community about the divisions that were taking place, those that were leaving were referred to as the AntiChrist. John dubs them so because of the dispute about how Christ was to be understood (i.e. human vs divine) that caused them to leave. Mainly these dissenters (or antichrists) believed an emphasis on the divinity of Christ and either diminished Christ coming in the flesh or simply rejected Christ's humanity (This is a heresy called Doecetism; claiming Christ only 'seemed' to be human).
The funny thing about divisions in the church is that it is the work of the AntiChrist- Ha Satan, The Adversary, The Bad Guy, The Devil. The work of the AntiChrist is to undo what Christ has built. Divisions in the church are evidence of the that (Not all mind you).
This is why John reminds the community....you are not like that. Some of those who remained may have considered joining the other faction. Therefore, John reminds them that they have already been given eternal life through Christ, they did not need a new teaching. They need only abide in the teaching they have been given through the Holy Spirit....That Christ is both divine and human and that is a mystery to be believed not explained.
Lord, keep us steadfast in our faith. Remove us from division within your church, help us to see that our unity rests in you and not arguments about you - Amen.