Sadly in our day, not much of anything. Luther, of course, began the Reformation by posting his 95 theses. His chief concern was the sale of indulgences. Underscoring that concern were two principle concerns- the singular authority of the Bible, and the doctrine of justification by faith alone. Luther, along with the other magisterial Reformers, argued that the Bible is our alone ultimate authority in binding our conscience with respect to our faith and practice. It denied that the church provided either a compelling interpretation of the Bible, or a second source of infallible information. (For an outstanding exposition of this issue see my friend Keith Mathison’s The Shape of Sola Scriptura.)
On justification Protestants protested against what seemed, at the time of Luther’s posting his theses. to be Rome’s perspective that the way a man had peace with God was by trusting in the finished work of Christ, and cooperating with the means of grace as they were poured out by the sacraments. That seeming perspective, however, became crystal clear during the counter-Reformation, specifically at the Counsel of Trent. There Rome declared as settled canon law, that anyone who says a man is justified by faith alone, apart from the works of the law, should be damned. The heresy that was prior to Trent more practical, implicit and consequential became precise, explicit and unchangeable.
What then Protestants protest is the false authority of Rome and her false gospel. We protest not because we are complainers, grumblers, sticks in the mud. We protest precisely because of our dual love for Jesus Christ, and those who are not yet covered in His blood. We do not protest Rome for all she ever was, or ever said. Indeed we protest the notion that Protestantism is something novel. We protest the turning aside from the gospel once delivered. We protest the notion that we are a mere branch of or an offshoot from the true church. We are, insofar as we hold to the glorious gospel truth that we have peace with God through trusting in the finished work of Christ alone, the continuing church, the sons of Augustine, Athanasius, Anselm, the sons of the father of the faithful, Abraham.
We protest that Rome is not catholic, that she in fact shuts out the saints. We, however, are catholic, embracing all those who turn to the living Christ alone. We protest that guarding, defending, proclaiming justification by faith alone is not sectarian, narrow, nor divisive. It is instead a fulfillment of the command that we contend for the faith (Jude 3). We protest against squishy, feel-good ecumenism that imperils souls, that buys the love and respect of men and sells the wrath of God. We protest the beard-stroking, nuance exploring, subtlety affirming of those who refuse to remember that Rome damned and damns justification by faith alone with clarity, forthrightness and immutability.
We protest the notion that we who protest are hidebound, out of step, tilting at long since fallen windmills. We are fighting for the faith.