In 1543, John Calvin wrote a tract entitled On the Necessity of Reforming the Church, in which he laid out for Emperor Charles V the central reasons why Protestants were demanding reform. It is well worth reading in full. This excerpt gives a summary of why the Protestant Reformation had to occur.
“We maintain, then, that at the commencement – when God raised up Luther and others, who held forth a torch to light us into the way of salvation, and who, by their ministry, founded and reared our churches – those heads of doctrine in which the truth of our religion, those in which the pure and legitimate worship of God, and those in which the salvation of men are comprehended, were in a great measure obsolete. We maintain that the use of the sacraments was in many ways vitiated and polluted. And we maintain that the government of the church was converted into a species of foul and insufferable tyranny…
If it be inquired, then, by what things chiefly the Christian religion has a standing existence amongst us, and maintains its truth, it will be found that the following two not only occupy the principal place, but comprehend under them all the other parts, and consequently the whole substance of Christianity: that is, a knowledge, first, of the mode in which God is duly worshipped; and, secondly, of the source from which salvation is to be obtained. When these are keptout of view, though we may glory in the name of Christians, our profession is empty and vain. After these comes the sacrament and the government of the church, which, as they were instituted for the preservation of these branches of doctrine, ought not to be employed for any other purpose; and, indeed, the only means of ascertaining whether they are administered purely and in due form, or otherwise, is to bring them to this test. If any one is desirous of a clearer and more familiar illustration, I would say, that rule in the church, the pastoral office, and all other matters of order, resemble the body, whereas the doctrine which regulates the due worship of God, and points out the ground on which the consciences of men must rest their hope of salvation, is the soul which animates the body, renders it lively and active, and, in short, makes it not to be a dead and useless carcass.”