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 40:8 The 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. This is in accordance with the general sentiment in the psalm, that religion tends to promote prosperity, happiness, and length of days on the earth.