After rejecting the idea with "God forbid," Paul gives the reason in the form of a question. Living in sin is not just wrong for a believer; it contradicts who the believer now is. Barnes explains: "To be dead to a thing is a strong expression denoting that it has no influence over us." He paraphrases Paul's question: "How shall we, who have become sensible of the evil of sin, and who have renounced it by solemn profession, continue to practice it?" The question is meant to make the answer obvious.

The question answers itself: those who have died to sin cannot keep living in it. Paul then explains how believers died. They were "baptized into his death" and "buried with him by baptism into death" (Romans 6:3-4). "Our old man is crucified with him," and "he that is dead is freed from sin" (Romans 6:6-7). So Paul tells them, "reckon ye also yourselves to be dead indeed unto sin, but alive unto God" (Romans 6:11).

"God forbid" is a firm denial. Barnes notes that the Greek reads literally, "It may not be," and calls it "a strong denial of what is implied in the objection."

Commentators explain "dead to sin" in more than one way. Gill describes two senses: for justified persons, sin "is not imputed to them to condemnation and death"; for sanctified persons, "it is not their course of life." Matthew Henry uses a picture: the one who is dead to sin "is freed from the rule and dominion of it, as the servant that is dead is freed from his master."

This question shows what God has done for His people, not just what He asks of them. Paul does not say "try to die to sin." He says "we, that are dead to sin." God has already joined believers to Christ's death, and the rest of the chapter builds on that fact (Romans 6:3-11).

The command comes after the gift. Because believers have died with Christ, Paul can say, "Let not sin therefore reign in your mortal body" and "sin shall not have dominion over you" (Romans 6:12, 14). Henry says that in profession believers are "quite cut off from all commerce and communion with sin, as those that are buried are quite cut off from all the world."