One of my favorite writings from the prolific pens of the Reformers is John Calvin’s letter to Cardinal Sadolet. Sadolet had previously written to the city of Geneva in the attempt to convince its citizens to rescind their decision to align themselves with the Protestant Reformation and return to the communion of Rome. Calvin, who at the time had been exiled from Geneva, was nevertheless asked by the city to draft a response. The result was, in my opinion, one of the clearest and most powerful (even if strongly polemical) declarations of what drove the Reformers in their tireless efforts to reform the medieval church. The following excerpt gives us a taste:
Now if you can bear to receive a truer definition of the Church than your own, say in future that it is the society of all the saints which, spread over the whole world and existing in all ages, yet bound together by the doctrine and the one Spirit of Christ, cultivates and observes unity of faith and brotherly concord. With this Church we deny that we have any disagreement. Rather as we revere her as our mother, so we desire to remain in her bosom.
But here you bring a charge against me. For you teach that all that has been approved for fifteen hundred years or more by the uniform consent of the faithful, is by our rashness torn up and destroyed. Here I will not require you to deal truly and candidly by us (though this should be spontaneously offered by a philosopher, not to say a Christian). I will only ask you not to stoop to a mean indulgence in calumny, which, even though we be silent, must be extremely injurious to your reputation with serious and honest men. You know, Sadolet, and if you venture to deny it, I shall make it plain to all, that you knew but cunningly and craftily disguised the fact, not only that our agreement with antiquity is far closer than yours, but that all we have attempted has been to renew the ancient form of the Church which, at first distorted and stained by illiterate men of indifferent character, was afterwards criminally mangled and almost destroyed by the Roman pontiff and his faction.
I shall not press you so closely as to call you back to that form which the apostles instituted, though in it we have the only model of a true Church, and whosoever deviates from it in the smallest degree is in error. But to indulge you so far, I ask you to place before your eyes the ancient form of the Church as their writings prove it to have been in the ages of Chrysostom and Basil among the Greeks, and of Cyprian, Ambrose and Augustine among the Latins; and after so doing, to contemplate the ruins of that Church which now survive among yourselves. Assuredly the difference will appear as great as that which the prophets describe between the famous Church which flourished under David and Solomon, and that which under Zedekiah and Jehoiakim had lapsed into every kind of superstition and utterly vitiated the purity of divine worship. Will you here declare one an enemy of antiquity who, zealous for ancient piety and holiness and dissatisfied with the corrupt state of matters existing in a dissolute and depraved Church, attempts to ameliorate its condition and restore it to pristine splendour?
There are three things on which the safety of the Church is founded and supported: doctrine, discipline, and the sacraments; and to these a fourth is added: ceremonies, by which to exercise the people in offices of piety. Now in order that we may spare the honour of your Church as much as possible, by which of these things would you have us judge her? The truth of prophetic and evangelical doctrine, on which the Church ought to be founded, has not only in a great measure perished in your Church, but is violently driven out by fire and sword. Will you force on me in place of the Church something which furiously persecutes everything sanctioned by our religion, both as transmitted by the oracles of God, embodied in the writings of holy Fathers, and approved by ancient Councils? Where, I ask you, exist among you any vestiges of that true and holy discipline which the ancient bishops exercised in the Church? Have you not scorned all their institutions? Have you not trampled all the canons under foot?
The point that I want to draw out from this is simple. To be Reformed (which also means to be always reforming) is to be more truly Catholic, not of course in the sense of converting to Rome, but in the sense of confessing and proclaiming the apostolic faith once for all delivered in the Scriptures and attested by the “holy Fathers” and the “ancient Councils”. It means having a passion and zeal for the unity of the church, the body of Christ, not in the sense of subjugating everything under the purported vicar of Christ and successor of Peter, but in the sense of uniting all things under the only head of the body which is Christ alone. It means submitting to the authority of the Word of God, not in the sense of abusing the Reformation principle sola Scriptura as though it were nuda Scriptura, but in the sense of, as John Webster put it, learning to give ear to the gospel in communion with the “exegetical fellowship” of the church.
I recently interacted with a Roman Catholic who made the claim that the Reformers erred in setting out to split from the church and create one of their own. I told her that nothing could be further from the truth, as Calvin himself emphasized to Sadolet. The Reformers desired nothing more than the unity of the one church of Jesus Christ. But they also realized that such unity could not be held together by institutional authority (i.e. the papacy) if said authority deviated from the truth. As Calvin affirmed in the Institutes, Christ is the sole basis of the unity of the church, but Christ never comes to us bare, as it were, but only as he is clothed in the gospel. While Christ cannot be reduced to the preaching of the gospel, neither can he be separated from it, and therefore a defection from the gospel inevitably leads to division from Christ. And division from Christ can only lead to division between the church that continues to proclaim the gospel and the church that does not.
In sum, Calvin’s ultimate mission was not to destroy the old and rebuild the new, but to reform the existing to align it in greater continuity with, primarily, the Word of God, but also, secondarily, with the consensus of the fathers and the faith defended by the early church councils. He had no desire to innovate; rather he only sought to renew through a retrieval of the catholic past. That is to say, Calvin was Reformed in order to be more truly Catholic.
 John Calvin, 1954. ‘Reply by John Calvin to the by Cardinal Sadolet’ in Calvin: Theological Treatises, ed. J.K.S. Reid, Louisville, KY; London: Westminster John Knox Press. pp.230-232