Psalms 18:20 - Coke's Commentary on the Holy Bible

The Lord rewarded me according to my righteousness— In this and the five following verses, David declares his own integrity, and that he had not departed from, but conscientiously observed, the precepts and commands which God had given him by the law of Moses; and that therefore God, in the deliverance which he had vouchsafed him, and peaceably establishing him on the throne of Israel, had testified his approbation of him, and abundantly rewarded him. His behaviour to Saul was exemplary; and there is no instance in this period of his life that can be alleged against him, in which he violated the known precepts of religion and virtue, enjoined by that constitution which he was under; and therefore, conscious of his integrity, thus far, he glories and rejoices, that God, who was witness to it, had thus bountifully rewarded it.