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.”
It is a great blessing to obtain the Lord’s grace without the work of the law and penance, as if one were receiving some public honor without having earned it.
After demonstrating by the example of the patriarch Abraham that faith is older than the law, Paul quotes a further witness in support of his position, viz., David the prophet and king.
[AD 384] Ambrosiaster on Romans 4:6