The connection between the righteousness of good works and that of faith.
There is no doubt that good works which proceed from a purified conscience are pleasing to God. Recognizing in us His own righteousness, He can only approve and prize it.
We must however be very careful not to be carried away by a worthless trust in good works to the point of forgetting that we are justified only by faith in Christ. For before God, there is no righteousness through works except that which corresponds to His own righteousness. The person who wants to be justified by works, then, must do more than produce just a few good deeds. He must bring with him perfect obedience to the Law. And those who have outstripped all others and have progressed the most in the Law of the Lord are still very far from this perfect obedience.
Moreover, even supposing that the righteousness of God should content itself with a single good work, the Lord would not find in His saints this one good work done in such a way that He would praise it as righteous. For, although this may seem astonishing, it is indisputably true that there is not a single good work springing from us which is entirely perfect and not soiled by some stain or other.
This explains why we who are sinners and sullied by numerous stains of sin must be justified by something outside of ourselves. We need Christ, then, all the time, so that His perfection may cover our imperfection, His purity may wash away our stains, His obedience may wipe out our disobedience, and, finally, that His righteousness might be freely put to our account — and all this without any consideration of our works, whose value cannot be sustained before the judgment of God.
But when our stains — which otherwise contaminate our deeds before God — are covered in this way, the Lord no longer sees anything in these acts except complete purity and holiness. This is why He honors these acts with great titles and praises. He calls them righteous and treats them as such. He promises them an enormous reward.
In short, we must conclude that the great value of union with Christ is found not only in the fact that we are freely justified because of it, but also in the fact that our works are considered as righteous and are recompensed with an eternal reward.
John Calvin, Truth For All Time, pp. 34, 35