John Calvin, Institutes of Religion - 3.14.
11. We must strongly insist on these two things: That no believer ever performed one work which, if tested by the strict judgment of God, could escape condemnation; and, moreover, that were this granted to be (though it is not,) yet the act being vitiated and polluted by the sins of which it is certain that the author of it is guilty, it is deprived of its merit. This is the cardinal point of the present discussion. There is no controversy between us and the sounder Schoolmen as to the beginning of justification. They admit that the sinner, freely delivered from condemnation, obtains justification, and that by forgiveness of sins; but under the term justification they comprehend the renovation by which the Spirit forms us anew to the obedience of the Law; and in describing the righteousness of the regenerate man, maintain that being once reconciled to God by means of Christ, he is afterwards deemed righteous by his good works, and is accepted in consideration of them. The Lord, on the contrary, declares, that he imputed Abraham's faith for righteousness, (Romans 4:3) not at the time when he was still a worshipper of idols, but after he had been many years distinguished for holiness. Abraham had long served God with a pure heart, and performed that obedience of the Law which a mortal man is able to perform: yet his righteousness still consisted in faith. 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 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.)...
12. ... I answer, that the grace which they call accepting, is nothing else than the free goodness with which the Father embraces us in Christ when he clothes us with the innocence of Christ, and accepts it as ours, so that in consideration of it he regards us as holy, pure, and innocent. For the righteousness of Christ (as it alone is perfect, so it alone can stand the scrutiny of God) must be sisted for us, and as a surety represent us judicially. Provided with this righteousness, we constantly obtain the remission of sins through faith. Our imperfection and impurity, covered with this purity, are not imputed but are as it were buried, so as not to come under judgment until the hour arrive when the old man being destroyed, and plainly extinguished in us, the divine goodness shall receive us into beatific peace with the new Adam, there to await the day of the Lord, on which, being clothed with incorruptible bodies, we shall be translated to the glory of the heavenly kingdom.
13. If these things are so, it is certain that our works cannot in themselves make us agreeable and acceptable to God, and even cannot please God, except in so far as being covered with the righteousness of Christ we thereby please him and obtain forgiveness of sins. God has not promised life as the reward of certain works, but only declares, "which if a man do, he shall live in them," (Leviticus 18:5) denouncing the well-known curse against all who do not continue in all things that are written in the book of the Law to do them. In this way is completely refuted the fiction of a partial righteousness, the only righteousness acknowledged in heaven being the perfect observance of the Law.[emphasis added]