What shall we say then? Are we to continue in sin that grace may abound? By no means! How can we who died to sin still live in it? -- Rom. 6:1-2
You can almost hear the shock in Paul's voice. He is aghast! "You all think the purpose of grace is so you can sin?! Are you crazy? Why, oh why, would you return to that life Jesus freed you from?"
Paul goes on to describe the intensity of the battle a Christian will face, and the reality that we sometimes give in to sin, but the idea that we would "blame" or presume upon grace is unthinkable to him. The gospel of grace is what frees us from the power of sin and enables, empowers, and motivates a life of holiness.