The Catholic priest Martin Luther (1483-1546) fired the opening shot in Germany in 1517 in what later became known as the "Protestant Reformation." Luther originally intended to reform the Catholic Church. By 1520, however, he had broken with the Church and the Church with him. From there on, he and his followers wanted nothing to do with "the Romish Church."
From the beginning, the focus of Luther's attack was the Sacrifice of the Mass. The following description is from Francis Clark, S.J., Eucharistic Sacrifice and the Reformation, 1960:
"'At no point was Luther so violently opposed to the mediaeval system,' says Dr. Brilioth, 'as in his repudiation of the Romish doctrine of the sacrifice of the Mass. This was the spear-point of his assault' (Eucharistic Faith and Practice, p. 137). However much they differed from one another on other questions, all the Reformers were agreed on this. They did not claim to be discarding only a late-medieval error or a mere popular superstition, for they allowed that the Romanists' doctrine of the Mass-sacrifice, in the form that confronted them, had been prevalent in the Church for long centuries. 'What I am attacking,' declared Luther in 1520, 'is something deep-rooted and seemingly impossible to eradicate, since it has been established by the practice of so many centuries and approved by the consent of all men. We shall have to cast out the greater part of the books now in honour, and to change almost the whole face of the Church.'"
Link (here) to the full Catholic Insight piece.
Photo is of Fr. Francis Clark, S.J. (here)