Recall from two weeks ago the three evidences in this verse that justification is by faith alone apart from works. To the one who does not work . That rules out working as part of obtaining justification. Justification happens while we are still ungodly before there can be any good works. So there is no place for good works to become a cause.
Faith is credited as righteousness.
Not faith plus his works, or works instead of his faith, but simply his faith is credited as righteousness. That is, God views the complete dependence on Christ for righteousness as the receiving of that righteousness. If you depend on it alone, it's yours completely. That is why Paul is exulting here. This is an essential and glorious part of the gospel.