The word Antichrist occurs four times in the Bible. The first occurence tells us that there were many antichrists around in John's time in the first century. These antichrists were originally part of the first-century community of believers:

1 John 2 verses 18 - 19

18 Little children, these are the end times, and as you heard that the Antichrist is coming, even now many antichrists have arisen. By this we know that it is the final hour. 19 They went out from us, but they didn’t belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

The next occurence gives the added information that the Antichrist denies that Jesus is the Christ. The Antichrist also refuses to accept Bible truths about God and his Son, Jesus Christ:

1 John 2 verses 22 - 23

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 Whoever denies the Son doesn’t have the Father. He who confesses the Son has the Father also. 

Antichrist is related to false prophets in the third time we meet the word. Such people do not accept that Christ came in the flesh. John confirms that the problem is already around even in his day:

1 John 4 verses 1 - 3

1 Beloved, don’t believe every spirit, but test the spirits, whether they are of God, because many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit who confesses that Jesus Christ has come in the flesh is of God, 3 and every spirit who doesn’t confess that Jesus Christ has come in the flesh is not of God, and this is the spirit of the Antichrist, of whom you have heard that it comes. Now it is in the world already.

The last occurence of the Antichrist confirms what we have already seen:

2 John 1 verse 7

7 For many deceivers have gone out into the world, those who don’t confess that Jesus Christ came in the flesh. This is the deceiver and the Antichrist.

Antichrist is comprised of two words anti and Christ. The word anti means opposite or opposed to. So we are being told that this influence is opposed to Christ.

To summarise:

Antichrist is:

1. An influence which was around in John’s day

2. An influence which spread from among some of the believers

3. An influence which denies that Jesus is the Christ

4. An influence which denies that Jesus came in the flesh

5. An influence which denies the Father and Son relationship of God and Christ