Adam Clarke on Psalms 37:31

The law of his God is in his heart - The Lord promised that a time should come in which he would make a new covenant with the house of Israel; he would put his laws in their minds, and in their hearts he would write them. This is fulfilled in the case above.

None of his steps shall slide - His holy heart always dictates to his eyes, his mouth, his hands, and his feet. The precepts which direct his conduct are not only written in his Bible, but also in his heart.