For the word of the LORD is upright, and all His work is done in faithfulness.
The Word of the Lord is upright. First of all, in itself, and then because it makes upright, it straightens out the man who is bent in on himself and turns him upward so that he acknowledges and loves God by faith. All His works are done in faithfulness. First . . . they are faithful, or done in faithfulness, as He promised. And thus it corresponds to the upright Word ... because He does not promise something fictitiously or deviously, but with a straightforward and clear meaning. And thus the works promised in the Word are in faithfulness, so that they might demonstrate that the Word was upright, in that what it promised straightforwardly, it faithfully observed and accomplished. Second, the works of the Lord, those which the Lord wants to be done and which are done according to Him, with Him, and in Him through us, are done in faithfulness, that is, by faith. For we live and act by faith, or, in faith, that is, faithfulness, which is one of the twelve fruits of the Spirit (Galatians 5:22-23) . . .. Third, in faith, that is, His works are true or firm, works that must be believed, contrary to the works of hypocrisy, which are reedy, faked, and upright only in appearance, untrue and fictitious, like the wolf in sheep's clothing.
From First Lectures on the Psalms, on Psalm 33 (Luther's Works 10:154-55)