Paul enters upon a grand theme of his letter. Having shown us that in Adam we are dead but in Christ we are made alive, he now goes on to show us the consequences of our union with Christ.
God is glorified in His grace shown to sinners. ‘Should we then sin more to magnify God’s grace more?’, Paul asks. He goes on to show that this is impossible to the believer, since the old man, who was a slave to sin, died with Christ on the cross. Christ died to sin on the cross and therefore so did the believer. As Christ rose from the dead, so the believer has been raised to newness of life.
In practice, the believer still has to struggle with sin in this world and so Paul enjoins us to realise what is true of us in Christ and to reckon ourselves dead to sin but alive to righteousness in Him.