John Calvin Commentary Genesis 26:7

"And the men of the place asked him of his wife. And he said, She is my sister. For he feared to say, My wife. Lest, [said he], the men of the place should kill me for Rebekah. Because she was fair to look upon." — Genesis 26:7 (ASV)

And the men of the place asked him. Moses relates that Isaac was tempted in the same manner as his father Abraham, in having his wife taken from him; and without doubt he was so led by the example of his father, that he, being instructed by the similarity of the circumstances, might become associated with him in his faith.

Nevertheless, on this point he ought rather to have avoided than imitated his father’s fault; for no doubt he well remembered that the chastity of his mother had twice been put in great danger; and although she had been wonderfully rescued by the hand of God, yet both she and her husband paid the penalty of their distrust: therefore the negligence of Isaac is inexcusable, because he now strikes against the same stone.

He does not in express terms deny his wife; but he is to be blamed, first, because, for the sake of preserving his life, he resorts to an evasion not far removed from a lie; and secondly, because, in absolving his wife from conjugal fidelity, he exposes her to prostitution; but he aggravates his fault, principally (as I have said), by not taking warning from domestic examples but voluntarily casting his wife into manifest danger.

From this it appears how great is the propensity of our nature to distrust, and how easy it is to be devoid of wisdom in times of perplexity. Since, therefore, we are surrounded on all sides with so many dangers, we must ask the Lord to confirm us by his Spirit, lest our minds should faint, and be dissolved in fear and trembling; otherwise we shall be frequently engaged in vain enterprises, of which we shall repent soon, and yet too late to remedy the evil.