John Calvin Commentary 1 Corinthians 6:17

"But he that is joined unto the Lord is one spirit." — 1 Corinthians 6:17 (ASV)

He that is joined to the Lord. He has added this to show that our connection with Christ is closer than that of a husband and wife, and that the former, accordingly, must be greatly preferred over the latter, so that it must be maintained with the utmost chastity and fidelity. For if he who is joined to a woman in marriage ought not to have an illicit connection with a harlot, much more heinous would this crime be in believers, who are not merely one flesh with Christ, but also one spirit. Thus there is a comparison between greater and less.