John declares it is the “last hour” and the antichrist is coming (2:18). What does he mean by last hour? Most pop-evangelicals have a sense of antichrist from the Left Behind books and there are plenty of conspiracy theories out there proving your most hated political leader is The Antichrist. But every generation has had a group convinced they were living in the end times and every generation has named more than one enemy the antichrist.
Although this exact phrase doesn’t appear elsewhere in the New Testament, the idea of the last days is common. One characteristic of the last days is that false teachers will come (2 Tim 3:1; 2 Pet 3:3; Jude 18). Jesus taught many false christs (pseudochrist) would appear in the last days (Matt 24:24; Mark 13:22). There are other warnings against false messiahs in Jewish literature (see this post for examples).
Here John uses the title antichrist an expected way. Most think of antichrist as the leader of the rebellion against God in the future, but that is not how the word is used in the New Testament. The word does not appear in Daniel or Revelation to describe any leader coming in the future, and Paul calls the leader of the end times rebellion the “man of lawlessness” (2 Thessalonians 2, see this post). This alone ought to caution us against throwing around the term antichrist to condemn political enemies. That is NOT how the term was used in the Bible. In fact, the use here in 1 John is theological, a teacher who denies certain things about Jesus is an antichrist.
In 1 and 2 John, there are already many antichrists in the world. He is referring to those opponents who have left his congregation and are deny that Jesus is the Messiah, the Christ. According to John, they “went out” from his church, but they were never part of that church because they did not remain (2:19). Since they deny Jesus is the Christ, they are the antichrist, the false prophets and teachers Jesus warned his disciples were coming into the world.
John says the presence of many antichrists is proof this is the last hour. There is a fair amount of application to be drawn from this passage. Certainly we live in a time when there are any number of people who have bad ideas and theology concerning who Jesus is. Although there are no more heretics today than in any other time in church history, the heretics have more access to public media than any other time in history. Just as Paul would say in the last days scoffers will come scoffing, John is saying that in these last days people who oppose Jesus as the Messiah and the son of God are common and they are extremely dangerous.
For John, any teaching that does not understand Jesus correctly is dangerous. If one does not understand who Jesus was and what Jesus did on the cross correctly, then they are in danger of misunderstanding salvation and not having eternal life. If one were to ask John “how do I define Jesus correctly?” he probably would responds, “read my book, the Gospel of John.” Obviously John did not have copies of his book on sale in the lobby, but 1 John assumes the theology of the Gospel of John, which was written to explain that Jesus is the Christ and that by believing one can have life in his name (John 20:30-31).