Being filled with grace (getting what you don't deserve) often means downplaying the significance of sin. yet, Scripture tells us that we cannot continue to sin that we go on showing how gracious our God is. John describes to his community that living in the forgiveness of sin (Exactly what it is that Christ does) draws us into relationship with Christ (to know who he is).
Moreover, John is more concerned with the community not giving lip-service to being a Christian. John was concerned that we preach what we practice so that our actions and deeds match our theology (how we talk about who God/Jesus is).