Well, I think he had the same view as Luther, except more extreme. Luther claimed, among other things, that the papacy lost its authority because of its rejection of sola fide (“the doctrine on which the Church stands or falls,” he said), its involvement with political affairs, corrupt bishops and sometimes popes, simony, and so on.
Calvin entirely rejected the papacy, going straight to the identification of the pope as antichrist (not
the antichrist like the Beast in Revelation, but “antichrist,” simply meaning anyone who is against the faith of Christ). Remember, Calvin and Luther were not only religious figures but also political ones.
Calvin had a very strict, moralistic hand over Geneva when he ruled it. People were frequently exiled and one father was jailed for two weeks for not naming his son Abraham, but rather some New Testament name.
Yes, Calvin was more extreme. Luther kind of just modified Catholicism with St. Augustine’s ideas, St. Thomas Aquinas’s political ideas, and some earlier writings. Calvin rejected everything, calling any reverence of the saints whatsoever “idolatry” and really saw what he could do. He still has this effect from the grave. Calvinism, for some odd reason, is increasingly popular (
carm.org and
gotquestions.com, two of the most popular Protestant apologetics websites, are both Calvinist).
This is a good Catholic source.