Remember that John was possibly the last man still alive by this point who had walked and talked with Jesus. However, he is not high and lifted up in his own senses, but reaches out in deep affection for those to whom he writes: 'My dear children'. In his old age, unlike some elderly, who choose to keep away from youth, he has nothing but tenderness for those much younger in the faith than he. He is not scolding them for their sins; rather he seeks to love them into goodness. There is always the danger that we begin to think lightly of our past sin or that we make it to be everything in life. How many books of Christian testimony have you read where the part before the person gets converted is far more interesting than afterwards, almost that the person is looking somewhat wistfully at that time? John says two things about sin here: 1. It is universal, so anyone who claims never to have sinned is a liar. 2. There is forgiveness of sins through what Jesus Christ has done. It would be possible to use both of these statements as an excuse to take sin lightly. If it is universal then it is an inherent condition of being human, so why fuss? And if Christ has provided a way of forgiveness, then that is sorted, we don't have to worry. Again John has two points to make about those ideas: 1. Christians are people who have come to know God and stemming from that knowledge must come obedience. As Jesus stated, even the demons know that God is Lord over all and tremble, it doesn't make them Christians! 2. If we claim to live in God then it is essential that we live in the same way as Jesus lived. That doesn't mean that we move to Israel and preach to Jews: it entails seeking to have the same character as Christ: humble, servant-like, trusting and looking to God alone. John is unequivocal that knowledge involves obedience and union involves imitation: sin can never be taken lightly if both of these statements are true.