The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times. (Psalm 12:6)
God promises nothing in vain, or for the purpose of disappointing man. This, at first sight, seems a matter of small importance; but if any person consider more closely and attentively how prone the minds of men are to distrust and ungodly doubtings, he will easily perceive how requisite it is for our faith to be supported by this assurance, that God is not deceitful, that he does not delude or beguile us with empty words, and that he does not magnify beyond all measure either his power or his goodness, but that whatever he promises in word he will perform in deed.
David here records, that the words of Jehovah are pure; but those who while lying in the shade and living at their ease liberally extol by their praises the truth of God’s word, when they come to struggle with adversity in good earnest, although they may not venture openly to pour forth blasphemies against God, often charge him with not keeping his word. Whenever he delays his assistance, we call in question his fidelity to his promises and murmur just as if he had deceived us.
There is no truth which is more generally received among men than that God is true; but there are few who frankly give him credit for this when they are in adversity. It is, therefore, highly necessary for us to cut off the occasion of our distrust; and whenever any doubt respecting the faithfulness of God’s promises steals in upon us, we ought immediately to lift up against it this shield, that the words of the Lord are pure. (Commentaries)