Purpose statements are good. Going back to elementary school we learned that first sentence was so important to what came after. Like, "In the Beginning...."
Which begs the question....why is John stating his purpose for writing in chapter 2? The purpose is the same but the audience is varied. I am writing to you who are children, mature in faith, young in faith. John's concern is that all of God's children remember their experience of forgiveness. This forgiveness is the foundation of our fellowship with God and the building of community of faith. Each stage of discipleship builds upon one another (as children, as young in faith and as the mature in faith). Our growth in discipleship moves us out of immaturity and into maturity. God never wants us to stand still in our faith or in our need for forgiveness. The purpose of this letter is to grow in faith, through the experience of forgiveness from Christ, in order to build an authentic community where lives are shared in this experience regardless of where you might find yourself in the journey.