The letter opens with a speaker…er, multiple speakers, who start declaring all kinds of things. It's mainly stuff about Jesus, but hey, what did you expect?
The speakers say that, from the beginning, they've been able to hear and see and touch "the word of life." That's just a poetic way of saying that, in addition to being God and all that, Jesus was a flesh and blood human being (source, 1179).
They also announce that they know all about eternal life and what God revealed to the world in Jesus. Secret knowledge? Bring it on.
They're saying all this so that everyone can live in harmony with them as one big happy Christian community. Kumbaya, folks.
Basically, the message they need to deliver is this: God is a light. That'll come in pretty handy when the power goes out.
If you live your life for God, then you won't ever be in the dark, they say. But don't toss your flashlights yet—it's just a metaphor.
How do you know you're in the light? Easy. You're part of a community—specifically the same Christian community that the speakers are part of. Hey, God loves joiners.
And how do you know you're not in the light? Well, one clue is if you say you're not a sinner. That's a bad move because, honey, everyone's a sinner.
The good news is that if you admit your sins to God, he'll forgive you, and things will be all hunky dory. Hopefully you'll start being a better person, too.
But if you keep going on about how sinless and perfect you are, you're clearly not tight with God.