Antichrists are popping up everywhere

[Labster]

It looks like the earliest definition of antichrist that you get from John is essentially the obvious one: an antichrist is a person who denies that Jesus is the Christ.  Like an anticommunist is a person who denies that Marx has the right economic system.  It's just an opponent on an important issue.  It appears that the specific antichrists he was talking about at the beginning were Gnostics, a good portion of whom were saying stuff like "Jesus got the secret knowledge, and you can be exactly like him too if you know these mysteries" which is de facto denying that Jesus was special, the divine son of God.

The comparisons to Nero follow pretty closely from that -- not only does he deny the Christ, but has his own claim of divinity, which pretty obviously makes him an "alternative Christ" candidate.  So once the empire adopts Christianity, it's pretty clearly not having antichrist(ian) leadership. There really isn't much difference between antichrist and being anti-Christian. I think that we've gotten this whole concept of an Antichrist that is secret and big is all essentially an anti-Christian plot to make us see enemies, where we should be seeing neighbors and brothers who we should love.

It's really easy to spot an antichrist, then.  They're the people, when someone says to love the enemy, goes out and tells them that it's better to hate the enemy instead. There's no need for complicated theories when you can just look for people speaking against the commandment to love neighbors as themselves. Denying Jesus's main commandment is more or less equivalent to saying that Jesus was not anointed to issue that commandment.