While we tend to think of The Antichrist as a single future figure, there are already many antichrists among us. It has been the case from the beginning of the church.
Ian Hamilton’s study of John’s Letters is helpful. He notes that belief and behavior are inseparable, and that what you believe ought to shape how you live. That’s a big ouch to me, by the way. We see this ultimately playing out in 1 John 2:19 (see below) where John starkly comments on those who have left them.
It makes sense to me that after the church’s birth, the devil would bring in antichrists in order to oppress and oppose the fledgling believers. Hamilton tells us that these manifested the same spirit of unbelief as the Future Antichrist which the church had been warned of (2 Thess 2:1-12).
Notably, the first examples had been part of the church, and began their life there. These antichrists opposed the teachings of the apostle. Among other things, they denied the Resurrection, that God came in the flesh, that Jesus was God, and that He was one with the Father. See also John 17:20-24.
In more modern times, the spirit of Antichrist has brought in “progressive deceivers” who poison the well regarding inerrancy. They admit that Scripture contains God’s Word, but will subtly introduce the notion that not every doctrine taught by the Bible writers (or even Jesus) is truth. Some even teach that Jesus, in His humanity, sinned.
If Jesus sinned, He couldn’t be our Savior. The spirit of Antichrist denies Jesus’ role in salvation. It is anti-God, anti-Christ, and anti-gospel. Like the serpent in Eden, these antichrists want to exult themselves above God.
Scriptures
Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have appeared; from this we know that it is the last hour. They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.
But you have an anointing from the Holy One, and you all know. I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. 1 John 2:18-22
Brethren, join in following my example, and observe those who walk according to the pattern you have in us. For many walk, of whom I often told you, and now tell you even weeping, that they are enemies of the cross of Christ, whose end is destruction, whose god is their appetite, and whose glory is in their shame, who set their minds on earthly things. Phil 3:17-19
Warning to keep watch
Watch yourselves, that you do not lose what we have accomplished, but that you may receive a full reward. Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son. 2 John 1:8-9
Be of sober spirit, be on the alert. Your adversary, the devil, prowls around like a roaring lion, seeking someone to devour. But resist him, firm in your faith, knowing that the same experiences of suffering are being accomplished by your brethren who are in the world. 1 Peter 5:8-9
Maranatha!