Deuteronomy 27:20
Cursed be he that lieth with his father’s wife; because he uncovereth his father’s skirt. And all the people shall say, Amen. Cursed be he that lieth with his father’s wife; because he uncovereth his father’s skirt. And all the people shall say, Amen.
This curse belongs to the series pronounced from Mount Ebal, the mount of curses, after Israel had passed over Jordan on entering Canaan, while the blessings for the obedient were pronounced from Mount Gerizim. The solemn ceremony at Ebal and Gerizim was a covenant of vows declared on entering Canaan, with the mountains themselves standing as witnesses to those vows, which the people afterward gathered to renew before Joshua. God showed particular care for His forgetful people by commanding that the commandments be written on plastered stones and by having the blessings and the curses repeated aloud from two prominent positions, so that the advantages of obedience and the disadvantages of disobedience were clearly set before the people and heard.
The law was made known in this public way to impress the people and to make it familiar to them. After each declaration the whole congregation answered Amen, and by accepting the conditions they placed themselves under solemn responsibility to fulfill them. The directions were spoken by Christ from the enfolding pillar of cloud. They state plainly the blessings of obedience and express the principles of the law of the kingdom of God made specific, so that minds are not left in ignorance and uncertainty. They were uttered to keep the people from forgetting, as a precaution and decided practical reminder that obedience to the commandments of God is for present and future good, with obedience bringing prosperity and disobedience resulting in a curse.
The curse was given to deter from transgression and to encourage obedience, because God takes no pleasure in the death of the wicked but takes greatest pleasure in those who turn from sin. His government is a government of personal responsibility, where each one must stand for himself. The chapter lays a never-ceasing obligation on all whom God has blessed with life and health and temporal and spiritual advantages, expecting public acknowledgment when God specially favors His people so that the name of God will be glorified as a testimony that His Word is verity and truth. Its claim is just as binding and fresh in importance now as when given, because God's gifts are fresh and continual.