1 John 5:13-21
John is writing so that his readers might become Christians and know that they are safe. If we're not sure what it really means to be a Christian (or if we've forgotten!), we can fin it in verse 13. John says it's about believing in the name of the Son of God or in other words, believing Jesus was and is who he says he is.
If we're Christians, John says we can know for sure that we have been given life forever and also we can ask in prayer with greater confidence because we're asking our Father who's forgiven and accepted us. People who aren't yet Christians can pray too and God will hear them (he hears everything) but it's not the same if you haven't asked for forgiveness and there's sin separating you from God. Prayer changes when you become a Christian.
Sin changes when you become a Christian too. All sin leads to death but God offers free forgiveness and life (Romans 6:23). Christians still make mistakes and do the old sins they used to but it's totally different because when a Christian (forgiven) is sinning - it's not killing them! God has taken the sting of sin away through Jesus' death on the cross and his resurrection. This is how John can talk about sin that doesn't lead to death. ALL sin leads to death but what he's talking about is forgiven sin. Forgiven sin doesn't lead to death. Making mistakes and doing wrong is also totally different when you're a Christian. Still wrong (and we still need to grow out of it) but it's forgiven! We can still have life to the full and life forever!
John finishes this letter by ramming his point home. In short - 'Jesus is the truth so don't bother with fakes!' Why is he being so clear about it? Why is he being so exclusive? So arrogant? Because John knows that believing in the name of Jesus changes everything and it's the difference between life and death.
Have you asked Jesus for forgiveness and for change? You're allowed to! And you're allowed to keep asking too! And if you've done this, remember that as you grow to become more like Jesus, your sinful mistakes aren't killing you any more when you fall back! Jesus is life!