It is tempting to think of someone who has been deceived by the spirit of the Antichrist and has bought into the denial of Christ's deity as something other than human; they are not. They are flesh and blood people just like you and I. And but for the grace of God, none of us would have come to faith in the Lord Jesus Christ.
The phrase "of the spirit of the Antichrist," makes those so deceived sound like some kind of spawn from a hideous monster. And what would we do if we should encounter a hideous monster? We would run, of course. Instead of running from human beings, however, we are to think spiritually, Biblically, rather than superstitiously; we are to present the claims of the gospel of Jesus Christ to these lost souls. We are to "feed him" and "give him a drink" (Roms. 12:17-21).
If it should so come to pass that we are presented with the opportunity to proclaim the gospel, we are to take that opportunity; it may be a divine appointment. And furthermore, it may be that God will grant this person or these persons the gift of repentance, whereby they can "escape the snare of the devil" (2 Tim. 2:25-26).
In 1 Corinthians chapter 6, verses 9-10, the apostle reminds us that "the unrighteous" will not "inherit the kingdom of God." Then he proclaims, "And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God." So the unrighteous will definitely not inherit the kingdom of God but by the grace of God the unrighteous can become the righteous and indeed inherit the kingdom of God; being unrighteous is not necessarily the end of the matter for any sinner. God still "justifies the ungodly" (Roms. 4:5; 5:6).
This, then, would hold true for the Judaizers of the churches of Galatia. "Let them be accursed" except God grant them repentance. Paul's proclamation of cursing is not necessarily the end of the matter for those who preach another gospel. Paul is writing to the Galatians commanding them to repent and cling to the gospel and thus to the Lord Jesus Christ.
So, you are talking with a human being and you discover in the course of your conversation that he or she denies that Jesus has come in the flesh; yes, they are of the spirit of the Antichrist. They are blind because they have been blinded by the god of this age, this world. Yet even then, that blindness can be penetrated by the glorious light of the gospel that by the grace of God and by His grace alone that shines into darkened hearts.
Being that Scripture teaches that the unrighteous will not inherit the kingdom of God and immediately says "And such were some of you. But...," we cannot withhold the gospel from anyone ... even those of the spirit of the antichrist. By doing so we are complying with the word of God and giving our enemy food and drink that God will use to heap condemnation on some, repentance on others.