Genesis 20:9
Then Abimelech called Abraham, and said unto him, What hast thou done unto us? and what have I offended thee, that thou hast brought on me and on my kingdom a great sin? thou hast done deeds unto me that ought not to be done. Then Abimelech called Abraham, and said unto him, What hast thou done unto us? and what have I offended thee, that thou hast brought on me and on my kingdom a great sin? thou hast done deeds unto me that ought not to be done.
Abimelech summoned Abraham and confronted him for hiding that Sarah was his wife by calling her his sister, demanding what offense had prompted it and charging him with deeds that ought not to be done. His course was wrongful and brought a grave sin and guilt upon Abimelech and his kingdom, exposing them to danger and punishment, and it revealed his imperfect faith and lack of truthfulness in failing to trust God's protecting care. Abimelech, king of Gerar, knew the true God and acted with an honest heart and clean hands, yet he already knew that to take another man's wife was a great sin even without a recorded command expressly forbidding it. The encounter therefore shows that the moral law, and in particular Thou shalt not commit adultery, was already known, binding and in force in the patriarchal age long before it was spoken on Sinai, while Abraham's untruthfulness also violated the ninth commandment. God showed providential care by warning before punishment and sternly rebuking Abraham through Abimelech, because the sacredness of marriage came from creation itself and the seventh commandment only guards what marriage already possessed.
Abraham's Deception Confronted
Abimelech summoned Abraham and confronted him over concealing that Sarah was his wife by calling her his sister, demanding what he had done and what offense had prompted it, and charging him with bringing a grave sin upon him and his kingdom by deeds that ought not to be done. The confrontation belonged to Abraham's dealings with Abimelech during his gradual development of faith, and it exposed Abraham's deception as putting others at risk and as an instance of lack of truthfulness. Abraham's conduct was wrongful and wronged others, for it brought grave sin and guilt upon Abimelech and his kingdom and exposed them to danger and punishment, not upon Abimelech alone.
The episode reveals imperfect faith, since Abraham said Sarah was his sister because he did not trust God's protecting care. On two occasions he passed Sarah off as his sister, failing to trust God, and his conduct toward Abimelech stands as lack of truthfulness within his gradual growth in faith. The account shows God's providential care and interest in people, in warnings before punishment and in repeated lessons in faith. God used Abimelech as the instrument through whom Abraham was sternly rebuked, and Abimelech's dealings with Abraham and Isaac gave him and his people opportunity to encounter the Spirit of God.
Abimelech was king of Gerar during Abraham's sojourn among the Philistines, and he knew the true God while his people showed a higher regard for morality than the other nations of Palestine. He acted with an honest heart and clean hands, yet recognized that Abraham's deception had led him into danger of dire punishment for taking another man's wife. After confronting Abraham over exposing him and his kingdom to great sin by what should not be done, he sent Abraham away with gifts and goodwill.
Law Known Before Sinai
Abimelech already knew that to take another man's wife was a great sin and guilt against himself and his kingdom, and his rebuke shows he regarded adultery as wrong even though Abraham had told him Sarah was his sister. He recognized Abraham's conduct as wrongdoing that ought not to be done, showing adultery was known as sin in the patriarchal age. This knowledge existed even though no recorded divine command had expressly prohibited it to the early patriarchs. He spoke as one who had acted with an honest heart and clean hands yet had been led by deception into danger.
Abimelech understood the prohibition of adultery, and God directly instructed him about it in that chapter. Although he had acted unwittingly, he recognized he had been led into danger and that taking Abraham's wife had brought a great sin upon him and his kingdom. That act would have brought dire punishment upon him if carried further.
The episode therefore proves that the moral law was already known before it was later written out in full. In particular the law of the seventh commandment, Thou shalt not commit adultery, was already known, binding, and in force in Abraham's time, long before Moses and long before it was spoken on Sinai. The principle forbidding adultery was known before the law was given at Sinai, and Abimelech's rebuke that Abraham had brought a great sin on him and on his kingdom proves mankind in general knew it.
The confrontation is also an example of violation of the ninth commandment before Sinai, through Abraham's lack of truthfulness in concealing that Sarah was his wife. It is cited as showing violations of both the seventh and the ninth commandments before the law was proclaimed at Sinai. Abraham's conduct toward Abimelech in bringing a great sin upon him and his kingdom through deeds that should not have been done is identified as an instance of that untruthfulness.
The sacredness of marriage does not come from the Decalogue given at Sinai, for the seventh commandment only guards a sacredness marriage already had from creation. As the fourth commandment guards the Sabbath, so the seventh guards the sanctity of marriage known and binding in the patriarchal age before Sinai. A patriarch's violation of that institution does not cancel its moral force, even when his conduct is called a grave wrong and a great sin against another and his kingdom.