I love the logic of what John says about the light and darkness, truth and lies. It's not difficult to see the sense in what he's saying if we take a minute to work it through...
John's met with Jesus and he's talking about him. He's seen, heard and touched eternity and he's passing on the message that Jesus is the light. Let's think through what John says about this. Light dispels darkness. Switch on a light in a dark room and all the darkness goes instantly. You can't have both at the same time. It's the same with Jesus. He is the perfect bright light (perfect/righteous/pure) and there can be no darkness (wrongness/sin) in him.
Then John talks about people. If we claim that we're friends of Jesus or Christians or whatever you want to call it, then it follows that our lives should be fairly well 'lit up'. If we're still fumbling around in darkness, making the same old dim mistakes, how can we say we're friends of Jesus? Our habits should be changing because if we've switched the light on, how can our lives still be dark? It's about having a REAL relationship with Jesus, the light of the world, rather than just pretending that we're anything to do with him. It's impossible to encounter Jesus at all and not change! If we say we're Christians and our lives don't look different, John's basically saying that we're liars!
Conversely, if we ARE walking in the light and we DO have a real relationship with Jesus, then not only will our lives be increasingly well lit up but we will be able to see each other more clearly too and our relationships with others will be lighter and better because all the secrets are gone!
John says we should fess up and be honest about our sin, our mistakes, our darkness. And if we do this there's an AMAZING promise from God (and he never breaks them!). If we own up to our sin (mistakes, darkness, badly drawn Easter cards, etc!) then God absolutely promises to forgive us AND more than this, he promises to purify us. That means he'll give our conscience a good scrubbing too so we don't need to feel guilty any more! Absolutely not! This means we can be full of light (full of Jesus!) once more and be walking in light continually, even though we will make mistakes again and again. We don't need to be stumbling around lost in the dark. It's about confession and getting real with God. It RULES! He offers a completely fresh start and the light to walk on into. Let's make sure we do it and not lie to ourselves any more!