'John', who is certainly not the same 'John' as who wrote the gospel of 'John' swears up and down he's telling the truth that he knew Jesus and by extension god and now he wants to pass along their message of light. He's also a huge talk-downer, constantly referring to his followers as little children. He informs us that we've been purged of all our sins by the blood of Christ, but then tells us we all sin, and to deny that is to lie. So the answer is to confess and be forgiven.
Try not to sin. But when you inevitably do, confess and all will be well. You know Jesus if you keep his commandments. Lots of people claim to know him and not follow his teachings, but they're liars. How else can you tell if you're a believer? Well, if you love your brother, you're good. If you don't: hell. Also, don't love the world, or indulge your lusts or be proud, because this is all ending soon and the only thing that will matter is whether you obeyed god.
It's the end of days, and there are a lot of antichrists about, which 'John' defines as former believers and non-believers. Anyone who doesn't believe in both Jesus and god doesn't worship the same god, so take that, Jews! Now, sit back and get ready for the return of Jesus.
The world ignores the christians, but that's okay because it will all be over soon. Meanwhile, keep hope and don't sin. Except that somewhere between chapter one and now, John has had a change of heart and decided that Whosoever abideth in him
sinneth not: whosoever sinneth hath not seen him, neither known him: whosoever sinneth hath
not seen him, neither known him (v. 6) In fact, the only people who sin are devils. He moves the goalposts even further, telling is that not only do christians not sin, they don't even do things that aren't righteous. So love one another, even if the world hates you. Hating your brother is akin to murder. Show your love through works. God knows what's in your heart, so all you have to do is ask for things and he'll give them.
Don't hedge your bets by worshiping every god around, you only need one. How will you know who to believe? If they say Jesus was flesh and blood, they're real. Also, the spirit is stronger in you than it is in the world. Anyway, love each other, because god is love (v. 8) and God sent
his only begotten Son into the world, that we might live through him (v. 9). If you love people god will live in you. If you believe in Jesus, god will love you. If you have fear, god doesn't love you, nor does he love you if you hate your brother.
We know Jesus was god's son because he was baptised and then he died on the cross. Also, the father, the son and the holy ghost are the same thing and you have to believe in all three. There is something called a sin unto death (v. 16), but what that might be, John doesn't say. Also, it's possible to commit sins that don't carry the death penalty, but again, we don't know what those are. Fortunately if you're a christian, you can't sin, just all those other assholes out there.