Even as David also describes the blessedness of the man, unto whom God imputes righteousness without works,
Read Chapter 4
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.” Commentary on Paul’s Epistles. ...
As David That is, David accounted a man happy in being justified by God's grace, and not by his own works, when he said: Blessed are they whose iniquities are forgiven by the mercies of God, and whose sins are covered; that is, covered so as to be no more, even in the sight of God. (Witham)