Deuteronomy 27:23

Cursed be he that lieth with his mother in law. And all the people shall say, Amen.

The prohibition of a man lying with his mother-in-law is placed among the curses. Those curses were to be pronounced from Mount Ebal, the mount of curses, and the blessings upon the obedient from Mount Gerizim, the mount of blessings, on the day Israel crossed over Jordan and passed into the land. God gave exact directions for the ceremony with particular care because Israel was forgetful, commanding that a rock be plastered and the commandments of God written on it and that the blessings and the curses then be repeated from the two prominent positions. This careful making known was to impress the law on the people and make it familiar because of its great importance. After each declaration the whole congregation was to assent, thereby accepting the conditions and placing themselves under solemn responsibility to fulfill them.

The verse belongs to the solemn covenant vows associated with Mount Ebal and Mount Gerizim that the people renewed at Shechem before Joshua's death; beyond that setting no further explanation of this particular curse is given. The directions were spoken by Christ with His own voice from the enfolding pillar of cloud, so they would not be forgotten. As part of Deuteronomy 26-28 and Deuteronomy 27, which plainly state the blessings of obedience, the verse expresses principles of the law of the kingdom of God made specific so that minds are not left in ignorance and uncertainty.

God gave the paired blessings and curses to set before the people the advantages of blessing and obedience and the disadvantages and consequences of wrongdoing. They were given to deter from transgression and encourage obedience, and as a warning against disobedience to His law. Obedience to the commandments is for present and future good and brings blessings and prosperity, while disregard and disobedience result in a curse. God takes no pleasure in the death of the wicked but takes greatest pleasure in those who turn from sin. The verse lays a never-ceasing obligation on all whom God has blessed with life and health and temporal and spiritual advantages to publicly acknowledge His special favor, so that the name of God will be glorified as a testimony that His Word is verity and truth. The message has not grown weak because of age; God's claims are just as binding now and just as fresh in importance as His gifts are fresh and continual.