1 Corinthians 6:16

Joined. In marriage a man and his wife are one in a lawful, holy union (Gen. 2:24), but in fornication a man and woman become one in an unlawful, unholy union. Only when it is according to God’s law can the union of the sexes be holy. The Corinthian believers would not question the statement that they were members of Christ, but they might doubt that by an act of fornication they would lose their high estate and become members of a harlot. They might be inclined to say that this would be an exaggeration of the effect of a moral lapse. Paul’s reasoning, based on Scripture, could not be successfully challenged.