Charles Ellicott Commentary Genesis 20:5

"Said he not himself unto me, She is my sister? And she, even she herself said, He is my brother. In the integrity of my heart and the innocency of my hands have I done this." — Genesis 20:5 (ASV)

In the integrity of my heart ... — Not only does Abimelech assert this, but Elohim admits the plea. And yet this Philistine king indulges in polygamy, and claims the right of taking the female relatives of anyone passing through his territory to add them to his harem. But the words mean only that he was not consciously violating any of his own rules of morality. This illustrates the Gospel principle that people will be punished not by an absolute decree, but equitably, according to their knowledge (Luke 12:47–48).

Abimelech was doing wrong, and was suffering punishment, but the punishment was remedial, and for his advancement in knowing right and doing right. It is thus by means of revelation that people have attained a proper understanding of the moral law. Though often called "the law of Nature," yet Nature does not give it, but only acknowledges it when given. The inner light is only a faint and inconstant glimmering, but Christ is the true light, for only by Him does the law of Nature become a clear rule for human guidance (John 1:9; Romans 2:14–15; Matthew 6:23).