History of Christianity
In the early 16th century, movements were begun by two theologians, Martin Luther and Ulrich Zwingli, that aimed to reform the Church.  Unlike earlier reformers  they considered the root of corruptions to be doctrinal (rather than simply a matter of moral weakness or lack of ecclesiastical discipline) and thus they aimed to change contemporary doctrines to fit their idea of the "true gospel".  The Protestant Reformation is so called because the movement's leaders "protested" against the ecclesiastical hierarchy and the pope, essentially electing to institute their reforms separately from it.  The term "Protestant", however, was not originally used by these leaders; instead, they called themselves "evangelical", emphasizing the "return to the true gospel (Greek: euangelion)".