Article by Michael Horton, the J. Gresham Machen professor of apologetics and systematic theology at Westminster Seminary California (Escondido, California), host of the White Horse Inn, national radio broadcast, and editor-in-chief of Modern Reformation magazine. He is the author of many books, including The Gospel-Driven Life, Christless Christianity, The Christian Faith, Calvin on the Christian Life, and Core Christianity: Finding Yourself in God’s Story. (original source here)
John Calvin felt the sting of the Devil’s taunt to Luther, “Are you alone wise among men?” We are certain of the gospel because it is so clearly revealed in Scripture—in contrast with the teachers of Rome.
I do not dream, however, of a clarity of faith which never errs in discriminating between truth and falsehood, is never deceived, nor do I figure to myself an arrogance which looks down as from a height on the whole human race, waits for no man’s judgment, and makes no distinction between learned and unlearned.
Indeed, it is better to suspend judgment than to rashly criticize and raise dissent. “I only contend that . . . the truth of the word of God is so clear and certain that it cannot be overthrown by either men or angels.” 
The Reformed have no controversy at all with the true Catholic church, Calvin contends.  “You know, Sadoleto,” he daringly presses, “that our agreement with antiquity is far closer than yours” and that we are only trying to “renew that ancient form of the church” that has been “distorted by illiterate men” and “was afterwards flagitiously mangled and almost destroyed by the Roman Pontiff and his faction.” 
Every aspect of the church’s ministry—its doctrine, the sacraments, ceremonies, and discipline—had been profaned by Rome. “Will you obtrude upon me, for the Church, a body which furiously persecutes everything sanctioned by our religion, both as delivered by the oracles of God and embodied in the writings of the Holy Fathers, and approved by ancient Councils?” 
Even Calvin’s humanist sympathies were tested by the evangelical emphasis. In many ways, the Dutch humanist Desiderius Erasmus (1466–1536) was a founding father of both the Reformation and the Counter-Reformation. However, behind Erasmus stands the broader influence of the Brethren of the Common Life, also known as the devotio moderna (modern devotion). This is especially worth mentioning because I think contemporary evangelical spirituality bears more in common with this movement than with the Reformation.
Founded in the fourteenth century by Gerard Groote, the Brethren represent a mystical-pietist reform effort. Among their distinguished alumni were cardinals and a pope, as well as Erasmus, Luther, Bullinger, Anabaptist leaders like Balthasar Hubmaier and Hans Denck, and the founder of the Jesuits, Ignatius of Loyola. Everything turned on “the imitation of Christ,” which was the title of the devotional bestseller written by Brethren member Thomas à Kempis.
However, what set the Reformers apart was that they challenged the doctrine of the medieval church. For the most part, the Brethren were not interested in church doctrine and ritual, and they were generally inclined toward more optimistic views of free will and justification as inner transformation.
As he approached the fork in the road, Calvin declared, “I am a pupil of Luther’s.” Addressing Emperor Charles V, he said, “God roused Luther and the others, who carried the torch ahead, in order to recover the way of salvation; and by whose service our churches were founded and established.” 
Also like Luther, Calvin thought of justification not as merely one doctrine among many, but as the heart of the dispute with Rome. Of this doctrine he said,
“This is the main hinge on which religion turns . . . . For unless you first of all grasp what your relationship to God is, and the nature of his judgment concerning you, you have neither a foundation on which to establish your salvation nor one on which to build piety toward God.” 
All of the other abuses—pilgrimages, merits, satisfactions, penances, purgatory, tyranny, superstitions, and idolatry—flow from this fatal fountain of denying justification.
Calvin, “Reply by John Calvin to Cardinal Sadoleto’s Letter,” in Selected Works of John Calvin, 1:54.
5. Calvin, “The Necessity of Reforming the Church,” in Selected Works of John Calvin, 1:125.
6. Calvin, Institutes 3.11.1.