John Calvin Commentary Psalms 18:21

"For I have kept the ways of Jehovah, And have not wickedly departed from my God." — Psalms 18:21 (ASV)

For I have kept the ways of Jehovah. He had spoken in the preceding verse of the cleanness of his hands, but finding that men judged him perversely and were very active in spreading evil reports about him, he affirms that he had kept the ways of the Lord, which is equivalent to his appealing the matter to the judgment seat of God.

Hypocrites, it is true, are accustomed to confidently appeal to God in the same way; indeed, there is nothing they are more inclined to do than to trifle with the sacred name of God, using it as a cover to conceal their hypocrisy. But David brings forward nothing that men could not have certainly known to be true, if any regard for justice had existed among them.

Let us, therefore, from his example, endeavor above all things to have a good conscience. And, in the second place, let us have the magnanimity to despise the false judgments of men and to look up to heaven for the vindicator of our character and cause. He adds, I have not wickedly departed from my God. This implies that he always aimed directly at the mark of his calling, although the ungodly attempted many things to overthrow his faith.

The verb he uses does not denote only one fall, but a defection that utterly removes and alienates a man from God. David, it is true, sometimes fell into sin through the weakness of the flesh, but he never desisted from pursuing godliness, nor did he desert the service to which God had called him.