Let me give you one reason for this and an application. The reason Christianity must build our lifestyle on God’s mercy in Jesus Christ is that the whole point of Christianity is to glorify the mercy of God in Christ. We exist to put the glory of God’s mercy in Christ on display. So Paul spends 11 chapters opening the mercy of God in the work of Christ, and then says, “therefore” build your lives on this. Live a life that shows this ! That’s why you exist.