Kids. Do not sin. Please. Don't do it. There is determination here in what John is saying, the desperate voice of a loving father who sees his dearest children on the verge of doing something they will regret. John's whole purpose in writing is to persuade people to stop sinning, to not get tangled up again the world, to not wind up in darkened alleys instead of basking in the glorious light.
These is a touch of Paul here. Just because we will be forgiven, just because grace is ridiculous and outlandish and extravagant, don't risk it. (Romans 6.1-2) Don't test God. Don't get complacent. Don't take such an incredible salvation for granted.
And don't sin. Don't buy into the world's take on short term gratification: feel good now and worry about the consequences later. Do we cheapen the cross by hammering on about forgiveness unsteady of preaching about the danger of sin? Sin kills (Romans 6.23). Sin is toxic, addictive, poisonous. Sin alienates us from God and from each other. Sin is a very serious business indeed.
Why then is it still so attractive? We have died to sin and yet we still long after it. (Romans 6:2, 1 John 3:9) Why do I waste my time dabbling in what kills instead of celebrating the truth of the God who died for me to bring me fullness of life? (John 10.10) If eternal pleasures are with God, if fullness of joy is only found at his right hand, (Psalm 16.11) then why am I messing around with what I thought used to satisfy me but never really did?
It is, as C. S. Lewis puts it...
"It would seem that Our Lord finds our desires not too strong, but too weak. We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea. We are far too easily pleased."
Oh God, open up my eyes that I might see your beauty today. That I might know the worth of your son's sacrifice. That I would have eyes only for him. Let me not get side tracked...