Mercy, for a fanciful character, John cuts it to the cloth in this chapter.
He open up with “My little children…”, that sounds nice. John says we shouldn’t sin, but that if we do (and we know we will because he said in chapter 1 that anyone who says they have no sin is lying) that we have Jesus who advocates on our behalf, who has appeased the justice of the Father. This is good news (and it’s not just us, it’s available for the sins of the whole world.)
And then the hammer drops. We know that we have come to know Jesus if we keep his commandments. “Believe” must bring obedience and action, otherwise our belief is shallow (sounds like our man James, yes? Faith without works is dead.) If we say we know Jesus and we don’t keep his commandments John says we are lying (he’s quick to call out liars in this section). “Whoever says he abides in him ought to walk in the same way in which he walked”. John was the last living direct apostle, he likely knew everything written before him. Apparently at the end of the 1st century they’re having issues with people saying they believe in and follow Jesus who aren’t actually following him with their lives. John is having none of it.
Remember, though, that this doesn’t mean we can live a sinless life, John already made that clear. What he’s saying is, this ain’t no show. If you say you’re going to follow Jesus, you have to be doing what he did, doing what he tells you. If not, you might be hanging around the bros and wearing the clothes and reading the books but you’re not actually submitting to Jesus, which means your life as a Christian is a lie.
Then John breaks into talking about the “new” commandment being the same as the “old” commandment. This almost has to be love God with all your heart mind and soul and love your neighbor as yourself. Jesus says that sums up the law and the prophets and that continues to guide what we do today. That’s why John says here that if you hate your brother you are still in darkness, where those who love your brothers abide in the light (again, light and dark have connotations of the characteristics of God and us faithfully living those out.)
There’s kind of a neat little poem in the middle, pretty easy to get, though.
Then he moves into some stuff that sounds a lot like Galatians and James. He says, “Do not love the world or the things in the world. If anyone loves the world, the love of the father is not in him.” We have to pick who to serve, we can’t chase what the world honors because we know that Jesus didn’t chase that (in fact his life was a collection of shaming in the world’s perspective.) Ultimately, the world will pass away, but if we abide in God, He is faithful and we will be protected forever.
And then the antichrist(s) show up. That’s not capitalized because it’s not a specific person, the word basically means “those who oppose the Messiah”. Note John says there are many of them, they are around in John’s day and they are characterized as those who deny Jesus as the Messiah, or Jesus as the son and God as the father. And they apparently try to deceive others into denying those things as well (I’m looking at you, Jehovah’s Witnesses.) Even so, John says we don’t have to worry, stick with Jesus and the things we know about Him and we will be protected by the Son and the Father eternally. Stick with Jesus and it’ll be fine.