One Calvin’s most concise articulations of that understanding comes in his celebrated reply to Cardinal Sadoleto in September of 1539. The Cardinal had been trying to win back Geneva into the fold of the Roman Catholic church and wrote a letter arguing the Reformers, their theology, and their efforts. After an invitation to respond by some worried Genevans, Calvin replied while in exile in Strasbourg. At the center of his reply was his parry against Sadoleto’s charges at a Reformed understanding of justification:
You, in the first place, touch upon justification by faith, the first and keenest subject of controversy between us. Is this a knotty and useless question? Wherever the knowledge of it is taken away, the glory of Christ is extinguished, religion abolished, the Church destroyed, and the hope of salvation utterly overthrown. That doctrine, then, though of the highest moment, we maintain that you have nefariously effaced from the memory of men. Our books are filled with convincing proofs of this fact, and the gross ignorance of this doctrine, which even still continues in all your churches, declares that our complaint is by no means ill founded. But you very maliciously stir up prejudice against us, alleging that, by attributing every thing to faith, we leave no room for works.
I will not now enter upon a full discussion, which would require a large volume; but if you would look into the Catechism which I myself drew up for the Genevans, when I held the office of Pastor among them, three words would silence you. Here, however, I will briefly explain to you how we speak on this subject.
First, We bid a man begin by examining himself, and this not in a superficial and perfunctory manner, but to sift his conscience before the tribunal of God, and when sufficiently convinced of his iniquity, to reflect on the strictness of the sentence pronounced upon all sinners. Thus confounded and amazed at his misery, he is prostrated and humbled before God; and, casting away all self-confidence, groans as if given up to final perdition. Then we show that the only haven of safety is in the mercy of God, as manifested in Christ, in whom every part of our salvation is complete. As all mankind are, in the sight of God, lost sinners, we hold that Christ is their only righteousness, since, by his obedience, he has wiped off our transgressions; by his sacrifice, appeased the divine anger; by his blood, washed away our stains; by his cross, borne our curse; and by his death, made satisfaction for us. We maintain that in this way man is reconciled in Christ to God the Father, by no merit of his own, by no value of works, but by gratuitous mercy. When we embrace Christ by faith, and come, as it were, into communion with him, this we term, after the manner of Scripture, the righteousness of faith.
What have you here, Sadolet, to bite or carp at? Is it that we leave no room for works? Assuredly we do deny that, in justifying a man, they are worth one single straw. For Scripture everywhere cries aloud, that all are lost; and every mans’s own conscience bitterly accuses him. The same Scripture teaches, that no hope is left but in the mere goodness of God, by which sin is pardoned, and righteousness imputed to us. It declares both to be gratuitous, and finally concludes that a man is justified without works, (Rom. iv. 7.) But what notion, you ask, does the very term Righteousness suggest to us, if respect is not paid to good works? I answer, if you would attend to the true meaning of the term justifying in Scripture, you would have no difficulty. For it does not refer to a man’s own righteousness, but to the mercy of God, which, contrary to the sinner’s deserts, accepts of a righteousness for him, and that by not imputing his unrighteousness. Our righteousness, I say, is that which is described by Paul, (2 Cor. v. 19,) that God bath reconciled us to himself in Jesus Christ. The mode is afterwards subjoined — by not imputing sin. He demonstrates that it is by faith only we become partakers of that blessing, when he says that the ministry of reconciliation is contained in the gospel. But faith, you say, is a general term, and has a larger signification. I answer, that Paul, whenever he attributes to it the power of justifying, at the same time restricts it to a gratuitous promise of the divine favor, and keeps it far removed from all respect to works. Hence his familiar inference — if by faith, then not by works. On the other hand — if by works, then not by faith.
But, it seems, injury is done to Christ, if, under the pretence of his grace, good works are repudiated; he having come to prepare a people acceptable to God, zealous of good works, while, to the same effect, are many similar passages which prove that Christ came in order that we, doing good works, might, through him, be accepted by God. This calumny, which our opponents have ever in their mouths, viz., that we take away the desire of well-doing from the Christian life by recommending gratuitous righteousness, is too frivolous to give us much concern. We deny that good works have any share in justification, but we claim full authority for them in the lives of the righteous. For, if he who has obtained justification possesses Christ, and, at the same time, Christ never is where his Spirit in not, it is obvious that gratuitous righteousness is necessarily connected with regeneration. Therefore, if you would duly understand how inseparable faith and works are, look to Christ, who, as the Apostle teaches, (1 Cor. i. 30,) has been given to us for justification and for sanctification. Wherever, therefore, that righteousness of faith, which we maintain to be gratuitous, is, there too Christ is, and where Christ is, there too is the Spirit of holiness, who regenerates the soul to newness of life. On the contrary, where zeal for integrity and holiness is not in vigour, there neither is the Spirit of Christ nor Christ himself; and wherever Christ is not, there in no righteousness, nay, there is no faith; for faith cannot apprehend Christ for righteousness without the Spirit of sanctification.
Since, therefore, according to us, Christ regenerates to a blessed life those whom he justifies, and after rescuing them from the dominion of sin, hands them over to the dominion of righteousness, transforms them into the image of God, and so trains them by his Spirit into obedience to his will, there is no ground to complain that, by our doctrine, lust is left with loosened reins. The passages which you adduce have not a meaning at variance with our doctrine. But if you will pervert them in assailing gratuitous justification, see how unskillfully you argue. Paul elsewhere says (Eph. i. 4) that we were chosen in Christ, before the creation of the world, to be holy and unblameable in the sight of God through love. Who will venture thence to infer, either that election is not gratuitous, or that our love is its cause? Nay, rather, as the end of gratuitous election, so also that of gratuitous justification is, that we may lead pure and unpolluted lives before God. For the saying of Paul is true, (1 Thess. iv. 7,) we have not been called to impurity, but to holiness. This, meanwhile, we constantly maintain, that man is not only justified freely once for all, without any merit of works, but that on this gratuitous justification the salvation of man perpetually depends. Nor is it possible that any work of man can he accepted by God unless it be gratuitously approved. Wherefore, I was amazed when I read your assertion, that love is the first and chief cause of our salvation. O, Sadolet, who could ever have expected such a saying from you? Undoubtedly the very blind, while in darkness, feel the mercy of God too surely to dare to claim for their love the first cause of their salvation, while those who have merely one spark of divine light feel that their salvation consists in nothing else than their being adopted by God. For eternal salvation is the inheritance of the heavenly Father, and has been prepared solely for his children. Moreover, who can assign any other cause of our adoption than that which is uniformly announced in Scripture, viz., that we did not first love him, but were spontaneously received by him into favor and affection? —Calvin’s Reply to Sadoleto
I think Calvin’s said enough for himself. But, just in case, you may also want to consult this summary by Robert Letham of what the doctrine of union with Christ does to secure and link firmly our justification, sanctification, and glorification.