Rom 4:6

“And after proving this from Abraham, he introduces David also as giving his suffrage in favor of the statement made. What then doth David say? and whom doth he pronounce blessed? is it him that triumphs in works, or him that hath enjoyed grace? him that hath obtained pardon and a gift? And when I speak of blessedness, I mean the chiefest of all good things; for as righteousness is greater than a reward, so is blessedness greater than righteousness. Having then shown that the righteousness is better, not owing to Abraham's having received it only but also from reasonings (for he hath whereof to boast, he says, before God); he again uses another mode of showing that it is more dignified, by bringing David in to give his suffrage this way. For he also, he says, pronounces him blessed who is so made righteous.”

“Then when he says, as David also, he proves the conditional statement by a quotation from a psalm. First, he gives its sense; second, he presents its words, at blessed are they; third, he excludes a false interpretation, at this blessedness, then. He says: as David also terms the blessedness of a man to whom God reputes, i.e., confers, justice without works, i.e., gratis, without preceding works: not because of deeds done by us in justice (Titus 3:5). But man's blessedness is from God, as the Psalm says: blessed is the man who makes the Lord his hope (Ps 40:5). Hence, it is plain that he has glory before God who is justified not by the works of the law, as has been said.”

“Paul backs this up by the example of the prophet David, who says that those are blessed of whom God has decreed that, without work or any keeping of the law, they are justified before God by faith alone. Therefore he foretells the blessedness of the time when Christ was born, just as the Lord himself said: "Many prophets and righteous men longed to see what you see and to hear what you hear and did not hear it."”