I John 2

I think this chapter is actually kind of dense.  Rather than unpack the whole thing, I will note that two things stand out to me.

"the sins of the whole world" (v. 2)
The global evangelist mission of Christianity is something I have pondered lately.  In particularly, that Christ came for every human being on earth.  Earlier, we saw that disciples were to preach to the four corners of the earth.

Here, John explains why - Jesus was the atonement of sins for the whole world.


Antichrists (v. 18-19)

Much is speculated on the nature of the "antichrist" of Revelations.  Here, John provides a definition of antichrist where the person came out from group of Christians, but was not really a part of the group.  (v. 19)

Likewise, there is more than one "antichrist".  John states that "many antichrists" have come.  (v. 18).  My view of Revelations is that it emphasizes recurring difficulties that Christians will endure throughout history, rather than a focus on a one-time event as a prelude to the return of Christ.  The latter is certainly discussed, but the preceding chapters are not exactly a prelude to it, but rather what many generations will face.

Here, the Antichrist is viewed in the same way.  Although John states that "you have heard that the antichrist is coming", John emphasizes that many people fill this role.  In that, many have already come.  John does not clarify whether there will be a special Antichrist, more powerful than the other Antichrists.  But John's emphasis on multiple Antichrists deemphasizes the importance of a single, very powerful one, which seems to be a popular conception of the term.