And…more from the speaker: anyone who believes that Jesus is the Messiah is a child of God. That means if you love God, then you have to love his children (i.e., other believers), too.
Oh, and you also have to obey his commandments. But no worries, says the author; his commandments are pretty simple: believe in Jesus and love other people.
Having faith in these things is like a victory over the evil powers of the not-great world. It's like shining a light in the darkness. Flashlights ready…
The speaker tells us that Jesus came "by water and blood." That's just referring to Jesus' baptism (water) and death (blood).
He continues: three things tell us the truth about Jesus: the water (baptism), the blood (his death), and the Spirit (or, as you might better know it, the Holy Spirit).
God has told the world all about Jesus, and that outranks anything ordinary people have to say about him. Lesson: go right to the source.
But how do you know what God has told the world? Well, you have faith.
If you don't believe in God and Jesus, you're sorely lacking in the faith department, says the author.
It's simple, really: believe in all the things the community says about Jesus, and you'll live forever with God. But if you challenge that, you're in a whole lot of trouble.
Because this audience is so good at believing and loving, says the author, God is sure to listen to and grant their prayers.
He keeps doling out the advice: if you see a fellow believer committing a regular, run-of-the-mill sin, you can feel free to pray for them. But if you see someone committing the worst sin of all (which is not believing in what the community says God teaches), then don't even worry about praying for them. People outside the community are too far gone. Feel free to wave to them as they're shoved through the gates of Hell on the Judgment Day, though.
And there's more: children of God don't sin. The speaker just means that they don't stray from their faith and the community (though they can obviously commit other sins like stealing or lying and stuff like that).
But everyone in this community is God's child, so they're good. Other folks are living in the world and are listening to the evil one.
The letter ends with a quick reminder not to bother with idols. And… we're done.