We are all sinners - we all do things in our every day that hold us back from a full relationship with God and that separate us from those around us. And so no matter how Christian we think we are - or how faithful to our own religion we think we are - we must realize that we are all sinners. Even those of us who believe ourselves to be saved, realize that is is because of Grace and God's love that we are saved and not out of our own righteousness.
Therefore, churches are houses for saints but instead refuge for sinners. Sunday mornings is not a gathering for those deemed righteous but is instead a time for all of us to spend in communion with God - feeling God's love and striving to be better.
What sets us apart, when we proclaim our faith is that we are proclaiming that we acknowledge our sinful nature and are going to strive to live the best life possible - a life of recovery.