John Calvin Commentary Psalms 78:38

"But he, being merciful, forgave [their] iniquity, and destroyed [them] not: Yea, many a time turned he his anger away, And did not stir up all his wrath." — Psalms 78:38 (ASV)

Yet he, being merciful, expiated their iniquity. To show more fully that no means had succeeded in swaying the Israelites and causing them to return to a sound state of mind, we are now informed that, although God bore with their multiplied transgressions and exercised his mercy in forgiving them, they nonetheless manifested their wickedness by abusing his kindness whenever it was displayed, just as they had shown themselves rebellious and obstinate when he treated them with severity.

At the same time, the reason is given why they did not utterly perish. They no doubt deserved to face a common destruction, but it is declared that God mitigated his anger so that a remnant of them might remain. So that no one might infer from these examples of vengeance that have been mentioned that God had proceeded to punish them with undue severity, we are told that the punishments inflicted upon them were moderate—indeed, mild, when compared with the aggravated nature of their wickedness.

God kept back his hand, not looking so much to what they had deserved, as desiring to make way for his mercy. We are not, however, to imagine that he is changeable when at one time he disciplines us with a degree of severity, and at another time gently draws and allures us to himself; for in the exercise of his matchless wisdom, he resorts to different means by which to test whether there is really any hope of our recovery.

But the guilt of men becomes more aggravated when neither his severity can reform them nor his mercy melt them. It is to be observed that the mercy of God, which is an essential attribute of his nature, is here given as the reason why he spared his people, to teach us that he was not induced by any other cause than this to show himself so inclined and ready to pardon.

Moreover, since he pardoned them not only on one occasion, nor in one respect, it is affirmed that he expiated their iniquity, that he might not destroy them; and again, that although he had often been provoked, he yet did not cease to turn away his anger; and, finally, that he mitigated his chastisements, so that the people would not be overwhelmed by their weight.