Some in Corinth treated what they did with their bodies as unimportant. Paul answers with a question about who their bodies belong to. The first question reminds them that believers are joined to Christ, body included: their bodies are "the members of Christ." The second asks whether he should take what belongs to Christ and join it to a harlot. Barnes calls this an argument against sexual sin built on the believer's union with Christ. He asks whether what is "a part, as it were, of the pure and holy Saviour" should be given over to impurity. Gill calls the idea "most absurd, indecent, abominable, and detestable."

Paul answers his own second question at once: "God forbid" (ESV: "Never!"). He then explains why: "he which is joined to an harlot is one body," while "he that is joined unto the Lord is one spirit" (1 Corinthians 6:16-17). His conclusion is direct: "Flee fornication" (1 Corinthians 6:18).

"Members" here means parts of a body, like hands or feet. Paul uses the same picture later in the letter: "Now ye are the body of Christ, and members in particular" (1 Corinthians 12:27). The Corinthian church had already had a case of fornication reported among them (1 Corinthians 5:1). In 1 Corinthians 6:12-13 the ESV prints "All things are lawful for me" and "Food is meant for the stomach and the stomach for food" in quotation marks, as sayings Paul is answering.

This question shows how fully God claims His people. Christ does not save only souls; He joins believers to Himself so closely that their bodies are His members. Paul grounds this in God's action: "God hath both raised up the Lord, and will also raise up us by his own power" (1 Corinthians 6:14). The body has a future with God.

Matthew Henry's Concise Commentary draws the conclusion: "The body is for the Lord; is to be an instrument of righteousness to holiness, therefore is never to be made an instrument of sin." What Christ has joined to Himself is honored, not cheap.