John Calvin, Institutes of Religion - 3.14.
10. Even were it possible for us to perform works absolutely pure, yet one sin is sufficient to efface and extinguish all remembrance of former righteousness, as the prophet says, (Ezekiel 18:24.) With this James agrees, "Whosoever shall keep the whole law, and yet offend in one point, is guilty of all," (James 2:10.) And since this mortal life is never entirely free from the taint of sin, whatever righteousness we could acquire would ever and anon be corrupted, overwhelmed, and destroyed, by subsequent sins, so that it could not stand the scrutiny of God, or be imputed to us for righteousness. In short, whenever we treat of the righteousness of works, we must look not to the legal work but to the command. Therefore, when righteousness is sought by the Law, it is in vain to produce one or two single works; we must show an uninterrupted obedience. God does not (as many foolishly imagine) impute that forgiveness of sins once for all, as righteousness; so that having obtained the pardon of our past life we may afterwards seek righteousness in the Law. This were only to mock and delude us by the entertainment of false hopes. For since perfection is altogether unattainable by us, so long as we are clothed with flesh, and the Law denounces death and judgment against all who have not yielded a perfect righteousness, there will always be ground to accuse and convict us unless the mercy of God interpose, and ever and anon absolve us by the constant remission of sins. Wherefore the statement which we set out is always true, If we are estimated by our own worthiness, in every thing that we think or devise, with all our studies and endeavors we deserve death and destruction.[emphasis added]