From the additional notes to Charles Spurgon's The Treasury of David, Psalm 119.
Verse 66. — For I have believed thy commandments. These words deserve a little consideration, because believing is here joined to an unusual object. Had it been, "for I have believed thy promises," or, "obeyed thy commandments," the sense of the clause had been more obvious to every vulgar apprehension. To believe commandments, sounds as harsh to a common ear, as to see with the ear, and hear with the eye; but, for all this, the commandments are the object; and of them he saith, not, "I have obeyed"; but, "I have believed."
To take off the seeming asperity of the phrase, some interpreters conceive that "commandments" is put for the word in general; and so promises are included, yea, they think, principally intended, especially those promises which encouraged him to look to God for necessary things, such as good judgment and knowledge are. But this interpretation would divert us from the weight and force of these significant words. Therefore let us note, —
1. Certainly there is a faith in the commandments, as well as in the promises. We must believe that God is their author, and that they are the expressions of his commanding and legislative will, which we are bound to obey. Faith must discern the sovereignty and goodness of the law maker and believe that his commands are holy, just, and good; it must also teach us that God loves those who keep his law and is angry with those who transgress, and that he will see to it that His law is vindicated at the last great day.
2. Faith in the commandments is as necessary as faith in the promises; for, as the promises are not esteemed, embraced, and improved, unless they are believed to be of God, so neither are the precepts: they do not sway the conscience, nor incline the affections, except as they are believed to be divine.
3. Faith in the commands must be as lively as faith in the promises. As the promises are not believed with a lively faith, unless they draw off the heart from carnal vanities to seek that happiness which they offer to us; so the precepts are not believed rightly, unless we be fully resolved to acquiesce in them as the only rule to guide us in obtaining that happiness, and unless we are determined to adhere to them, and obey them. As the king's laws are not kept as soon as they are believed to be the king's laws, unless also, upon the consideration of his authority and power, we subject ourselves to them; so this believing notes a ready alacrity to hear God's voice and obey it, and to govern our hearts and actions according to his counsel and direction in the word. — Thomas Manton.