Calvin: Paul says For Believers No other Righteousness but Christ's, Salvation by Grace Alone!

"Hence we infer, according to the reasoning of Paul, that it was not of works. In like manners when the prophet says, "The just shall live by his faith," (Habakkuk 2:4) he is not speaking of the wicked and profane, whom the Lord justifies by converting them to the faith: his discourse is directed to believers, and life is promised to them by faith. Paul also removes every doubt, when in confirmation of this sentiment he quotes the words of David, "Blessed is he whose transgression is forgiven, whose sin is covered," (Psalm 32:1.) It is certain that David is not speaking of the ungodly but of believers such as he himself was, because he was giving utterance to the feelings of his own mind. Therefore we must have this blessedness not once only, but must hold it fast during our whole lives. Moreover, the message of free reconciliation with God is not promulgated for one or two days, but is declared to be perpetual in the Church, (2 Corinthians 5:18, 19.) Hence believers have not even to the end of life any other righteousness than that which is there described. Christ ever remains a Mediator to reconcile the Father to us, and there is a perpetual efficacy in his death, viz., ablution, satisfaction, expiation; in short, perfect obedience, by which all our iniquities are covered. In the Epistle to the Ephesians, Paul says not that the beginning of salvation is of grace, but "by grace are ye saved," "not of works, lest any man should boast," (Ephesians 2:8, 9.)

John Calvin. Institutes of Religion. 3.14.11