What are the two alternatives Paul presents in verse 21?
Paul contrasts two reigning powers in verse 21: the old Adam, or sin, which has reigned like a king over individuals and humanity, and God's grace, which is likewise to reign like a king. These are the two alternatives—sin's dominion versus grace's dominion, with grace proving greater where sin abounded.
Les explains that when Paul speaks of a reign, he wants us to think of a king. The old Adam has reigned like a king over us as individuals and over humanity as a whole, but even so, grace is to reign like a king. Les illustrates this with his basic training experience: young soldiers away from home would do things they never did in their hometown because nobody knew them. External restraints like surroundings, circumstances, parental influence, and community can inhibit even the unbeliever, but in a strange environment the old Adam has free course. This shows the old Adam abounds with sin, yet where sin abounds, God's grace is always greater. Thus the two alternatives are the reign of sin, or old Adam, and the reign of grace.