Charles Ellicott Commentary Genesis 20:11

"And Abraham said, Because I thought, Surely the fear of God is not in this place. And they will slay me for my wife`s sake." — Genesis 20:11 (ASV)

Surely the fear of God ... — Abraham’s general condemnation of the people had some excuse in the widespread depravity of the nations in Canaan, but was nevertheless unjust. Even regarding these nations, they were not utterly corrupt (Genesis 15:16), and both in Egypt and in Gerar the standard of morality was higher than Abraham supposed.

His difficulty was the result of his own imperfect faith. However, the fact that this artifice was arranged between husband and wife when starting on their long wanderings proves that they rather overestimated than underestimated the risks that lay before them.

The expedient was indeed a poor one, and shows that Abraham’s faith was not yet that of a martyr. Yet it also shows that both of them felt that Abraham might have to save his life by a means almost as bad as death. And thus, after all, it was no commonplace faith, but one as firm at root as it was sorely tried and exercised.