18 Children, it is the last hour, and as you have heard that antichrist is coming, so now  many antichrists have come. Therefore we know that it is the last hour. 19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. 

20-21 But you have been anointed by the Holy One, and you all have knowledge. I write to you, not because you do not know the truth, but because you know it, and because no lie is of the truth. 22 Who is the liar but he who denies that Jesus is the Christ? This is the antichrist, he who denies the Father and the Son. 23 No one who denies the Son has the Father. Whoever confesses the Son has the Father also. (1 John 2:18-23 English Standard Version)

It may surprise some readers to hear that John is the only writer in the New Testament who uses the word 'antichrist.' And though he refers to a specific antichrist who was to come, he says that many antichrists had already appeared on the scene. What was the first characteristic of an antichrist he mentions? (vs 19)

They were people who departed from the truth of God's word and separated themselves from true believers. They rejected what the apostles taught in favour of their own ideas. This explains why groups exist today such as Mormons and Jehovah Witnesses, as well as many mega church cult leaders. What truth were the antichrists of John's day particularly denying? (vs 22)

They were denying that the man Jesus was the divine Messiah. So they rejected the testimony God the Father had given concerning God the Son when Jesus was baptized, and a voice from heaven proclaimed “This is my beloved Son, with whom I am well pleased.” (Matthew 3:17) False teachers don't necessarily stop using the name of Jesus and other familiar words in the Bible, they just twist the truth of those words to mean something else. But what is the encouragement Christians can take from vs 20-21.

The Holy Spirit in a believers life will, as Jesus said to His disciples, “guide you into all the truth.” (John 16:13) This doesn't mean we can't be misled, or don't need the sort of help John was giving the people he was writing to, but it does mean that the Holy Spirit comes alongside those who are true followers of Christ and who want to be faithful to the end.

Now while John is the only one to use the word 'antichrist', it doesn't mean no other writer spoke of a similar scenario. In 2 Thessalonians 2:3-4 the apostle Paul wrote: “Don’t let anyone deceive you in any way (that the day of the Lord has already come), for that day will not come until the rebellion occurs and the man of lawlessness is revealed, the man doomed to destruction. He will oppose and will exalt himself over everything that is called God or is worshiped, so that he sets himself up in God’s temple, proclaiming himself to be God.”

Christians have legitimate differences on exactly what Paul meant, whether he was speaking of what would happen before the then existing temple in Jerusalem was destroyed, or whether it refers to a time when the antichrist referred to by John will make his appearance in the world with a particular hatred towards God. Many Christians hold this latter view and are expecting a dominant figure on the world stage who will demand the sort of obedience that only belongs to God. (We see that sort of thing already in countries like North Korea and China.)

The arrival of antichrist heretics in John's day signified that the world had entered into the last stages of God's purposes, and the final event will be the return of Christ Jesus. Paul went on in his letter to the Thessalonians to say that “'the lawless one' who will be revealed, will be overthrown by the Lord Jesus with the breath of his mouth, and destroyed by the splendour of his coming.” (2 Thessalonians 2: 8) In the meantime Christians must cling to the truth of the gospel, and to be careful of the lies that are spread by those who prefigure the antichrist.