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; Psalm 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.

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.

The law of his God is in his heart,.... Which may be understood of the moral law, which was written in the heart of Adam an his creation, and continued there during his state of innocence, but was greatly obliterated by sin: there are some remains of it in fallen man, even in the very Gentiles; who, though without the law, do some things contained in it, which shows the work of it to be written in them; though in some it is scarcely legible, having lost all distinction between good and evil: but in regeneration the law of God is reinscribed, according to the promise of the covenant of grace, Jeremiah 31:33; and such have a real knowledge of the spirituality and perfection of it; of the impossibility of justification by it; and of Christ being the fulfilling end of it: and they have a strong affection for it; they love it; and delight in it after the inward man, and serve it with their minds; in which lies part of their conformity to Christ, and is a branch of their character as good men; see Psalm 40:8; moreover, the word here used, signifies any doctrine or instruction, and is sometimes used for the doctrine of the Gospel, Isaiah 2:3, and may have this sense here, and the meaning be, that that has a place in the hearts or righteous men, dwells richly there, and works effectually in them; and particularly the doctrine of grace which relates to covenant interest in God; for it is the law or doctrine of "his God" that is in his heart;
none of his steps shall slide; not that he shall never slip nor fall; for that is supposed Psalm 37:24; but he shall never finally and totally go out of the paths of truth, holiness, and righteousness: the Lord keeps the feet of his saints, and orders their steps, and directs them in the way they should go, and preserves them in it; and enables them to walk uprightly, according to the rule of the divine word, and in all his commandments and ordinances; and to hold on and out unto the end: or the clause may be rendered, "it", the law, "shall not cause his steps to slide" (b); neither the law of God, nor the Gospel of Christ, but shall guide him in the right way, and be a lamp unto his feet, and a light unto his path.
(b) "non vacillat gressus ejus", Cocceius.

steps--or, "goings"--for conduct which is unwavering (Psalm 18:36).

Heart - His thoughts, meditations and affections are fixed upon it. Slide - Slide, or swerve, from the rule, from God's law.