John Calvin Commentary Romans 16:20

"And the God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you." — Romans 16:20 (ASV)

What follows, God shall bruise Satan, etc., is a promise to confirm them, rather than a prayer. He indeed exhorts them to fight courageously against Satan, and promises that they would soon be victorious.

He was indeed once conquered by Christ, but not in such a way as to prevent him from continually renewing the war. He then promises ultimate defeat, which is not apparent during the contest.

At the same time, he does not speak only of the last day, when Satan shall be completely bruised. Rather, since Satan was then throwing all things into confusion—raging, so to speak, with loose or broken reins—he promises that the Lord would soon subdue him and cause him to be trodden, so to speak, underfoot.

Immediately a prayer follows—that the grace of Christ would be with them, that is, that they might enjoy all the blessings that had been obtained for them by Christ.