Pure; or, sincere; without the least mixture of vanity or falsehood; and therefore shall infallibly be fulfilled. This he seems to add to answer an objection which might arise in some men's minds concerning what was last said. You tell us, The, Lord saith, I will set him in safety, &c.; but saying and doing are two things. They are so indeed in men, who oft speak rashly what they cannot perform, and deceitfully what they never intend: but all God's words are pure from all manner of dross; from all folly, or fraud, or uncertainty; he is holy and true in all his doctrines, threatenings, predictions, and promises. Tried in a furnace of earth, i.e. made of such earth or clay as was proper for and then usual in that work. See 1 Kings 7:46.
Related Commentaries of Psalms 12:6
Psalms 12:6
6 The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times.