The term "antichrist" is found only in I and II John. Notice I John 2:18: "Little children, it is the last hour; and as you have heard that the Antichrist is coming, even now many antichrists have come, by which we know that it is the last hour."
There were many antichrists in the days of the apostle John, the end of the first century AD. These antichrists pretended to be part of God's true church, but as John writes in verse 19: "They went out from us, but they were not of us."
What is an antichrist? A key is found in II John 7: "For many deceivers have gone out into the world, who do not confess Jesus Christ as coming in the flesh. This is a deceiver and an antichrist." An antichrist is a deceiver. But how does an antichrist deceive us, and what does it mean, "who do not confess Jesus Christ as coming in the flesh"?
The answer to both of these questions can be given concisely as, "Antichrists deceive by teaching a false, unbiblical Christ." In essence, they change the nature of God to suit their own ends and preach it as truth. For instance, in John's day Gnosticism was sweeping the Roman Empire, combining Christianity, Jewish mysticism, and pagan philosophy and practices. One of many Gnostic-Christian strains was Docetism, a belief that Jesus was a human, but Christ was a divine spirit that came upon Jesus at his baptism in the form of a dove (see Luke 3:21-22). According to the Docetists, this Christ-spirit left Jesus before He suffered and died, for a basic Gnostic tenet is that physical flesh is tainted and impure, while spirit is pure and holy. Thus John warns his audience against ministers who teach this Docetist doctrine.
Paul shows in Galatians 1:6-9 that a different gospel was being preached already in the early AD 50s. Changing the gospel is another ploy of antichrists, obscuring the glorious good news of the Kingdom of God. This, too, changes the nature of God—that He is at work expanding His Family through converted, changed humans—as well as altering the beliefs and hope of those whom God is calling to His way of life. This deception, usually focused on doing away with God's law (see Matthew 7:21-23; Romans 3:31; 7:12-14, 22-25; etc.), has continued ever since. The apostle Paul calls this "the mystery of lawlessness [that] is already at work" (II Thessalonians 2:7).
We can also get a sense of what an antichrist is by breaking the word down into its component parts, anti- and Christos. Anti- is a Greek preposition that means "against." Christos, of course, is the title of Jesus, meaning "anointed one." Thus, an antichrist is simply one who is against the true Christ! The apostle John uses the term to describe a person who is hostile to true Christian interests, especially those false teachers who have come from within the church and are engaged in teaching error.
However, it is clear from other passages that there will be one particular Antichrist, who will appear on the world scene shortly before the return of Jesus Christ (see Revelation 13; 17; 19:17-21; Daniel 11:29-45; II Thessalonians 2:1-12).