David is not here speaking of the character in which God acts towards mankind in general, but what his own children find him to be. We have already seen in Psalm 18:26, that he is stern and severe towards the obstinate and rebellious; and even though he act with kindness towards them, in mercifully exercising forbearance towards them notwithstanding their iniquity, yet we find, that so far from seeking their full enjoyment in him, and trusting to his promises, they have no sense of his goodness. Nay, as soon as any adversity befalls them, they either become passionate and fretful, accuse God of acting cruelly towards them, or else complain that he is deaf to their prayers; and when they enjoy prosperity, they despise and neglect him, and as much as they are able flee from his presence. David, therefore, in speaking of the mercy and faithfulness of God, justly describes them as a treasure peculiar to the godly; as if he had said, We have no reason to be afraid that God will deceive us if we persevere in his covenant. These words, covenant and testimony, are of the same import, unless that the second is added as an explanation of the first. They comprehend the whole doctrine of the law, by which God enters into covenant with his chosen people.

And here’s the Westminster Larger Catechism:

Q. 101. What is the preface to the Ten Commandments?
A. The preface to the Ten Commandments is contained in these words, I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage. Wherein God manifesteth his sovereignty, as being JEHOVAH, the eternal, immutable, and almighty God; having his being in and of himself, and giving being to all his words and works: and that he is a God in covenant, as with Israel of old, so with all his people; who, as he brought them out of their bondage in Egypt, so he delivereth us from our spiritual thraldom; and that therefore we are bound to take him for our God alone, and to keep all his commandments.

Q. 153. What doth God require of us, that we may escape his wrath and curse due to us by reason of the transgression of the law?
A. That we may escape the wrath and curse of God due to us by reason of the transgression of the law, he requireth of us repentance toward God, and faith toward our Lord Jesus Christ, and the diligent use of the outward means whereby Christ communicates to us the benefits of his mediation.

Q. 154. What are the outward means whereby Christ communicates to us the benefits of his mediation?
A. The outward and ordinary means whereby Christ communicates to his church the benefits of his mediation, are all his ordinances; especially the word, sacraments, and prayer; all which are made effectual to the elect for their salvation.