Paul wants it to be clear: Just because we are justified by grace doesn’t mean our behavior doesn’t matter. 
He writes in order to continue to clarify our understanding of who we are through faith in Jesus & to empower us in our battle with sin. 

Our attitudes & actions matter in terms of our eternal destiny. If we exhibit a lifestyle that doesn’t reflect God as master of our daily lives, we should NOT be confident that we belong to him. BUT if we have genuine faith in Christ, we will joyfully obey Him.