Psalms 37:31

The law of his God is in his heart -That is, he loves it; he thinks of it; he makes it the inward rule of his conduct:Deuteronomy 6:6;Psalms 40:8The word “law” here is a general term for the truth of God - for all that he has revealed to guide men. As long as that truth is in the heart; as long as it is the object of love; as long as it is suffered to guide and control us, so long will our words and conduct be right. None of his steps shall slide -Margin, “goings.” The idea is, that his course will be firm and steady. He will not fall into sin, and his life will be prosperous and happy.