Deuteronomy 22 and 1 Corinthians 5 both address a specific divine prohibition for the morality of believers:
Deu 22:30 A man shall not take his father’s wife, nor discover his father’s skirt.
1Co 5:1 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father’s wife.
A comparison of these passages shows that God’s moral requirement for today’s believers who are under grace is no different than what is was for believers in Him who were under the Law—a man must not have intimate relations with his father’s wife! God’s moral standard for His people concerning this prohibition has not changed at all.
In fact, this comparison shows that even Gentiles who live in the age of grace know not to practice such fornication. Christians who assert that being under grace means that such sexual prohibitions in the Law no longer apply to God’s people have a wrong understanding of what being under grace means for Christian morality.