Hæc scripsi vobis de his, qui seducant vos.
I write you these things about those who would deceive you.
John, writing in the mid 70s (ish) of the 1st Century is already aware of Antichrist and those who would deceive Christians out of their faith. Whoever denies that Jesus is the Christ is the Antichrist.
Now, I don't mean horror movies or Satan Incarnate. Antichrist means "instead of" or "another" Christ. Yes, "Against" is implied. But it's more subtle. Whoever denies Jesus is the Messiah is, essentially, setting himself up as that anointed of God instead of Jesus. How is this? The Church says Jesus is the Christ, the Son of the Living God, who came into the world to save sinners. If I say he is not that, that is not simply another opinion on the matter. I are presuming to judge the man that the Church says is God, incarnate in the flesh, born of a virgin, fulfilling all prophecies, destroying all divisions, ending sin. And I am judging that to be not so. Since the Church is describing the Truth: I'm saying I know better. There is only one person who could know better: the Christ.
This spirit is already in the world.
Yes, it is always prompted by Satan, but it is present even in the most "innocent" of conversations: yes, I'm Catholic, but I believe in the right to abortion. Or you should get a divorce. We're Catholics, but you can do whatever you want. Vatican II says I can make up my own mind.
Antichrist come into the world.
Let what you heard from the beginning remain in you.
Those "you" words are plural. So, "What the Church has heard from the Beginning should remain in the Church". Even if your RCIA instructor tended to the wonky extremes of the liberal or conservative sides, always refer back to what the Church has heard from the beginning. Anything else is not some B-horror-movie monster, but "another Christ". And, really: that is the same thing.