I’d argue it started with the Avignon papacy and continued to worsen through the Western Schism and of course the string of morally ambiguous to downright abhorrent popes in the late 15th and early 16th centuries. There were of course a few very bad popes before these eras such as Stephen VI, Benedict IX, Sergius III. And there were some good popes during the era I mention, but the bulk of the protestant issues with the papacy can be traced back to the early 1300’s-mid 1500’s time frame, particularly the tail end of that time when there was a string of over a half dozen bad popes back to back. It was a time when there was a disproportionate amount of abuse from the papacy from Popes like Sixtus IV Innocent VIII, Alexander VI, Julius II, Leo X, Clement VII, which led significantly to the weakening of the office both temporally, morally and spiritually.
Now which pope broke the camel’s back specifically varies so to speak, but leading into the Protestant reformation it was a string of abuse by multiple popes that opened the door to the reformation. Now that’s not to say that the gates of hades prevailed against Christ’s overall church. The reformers felt their pulling away was helping resist the gates of hell. And if anything the Reformation helped the Catholic church to finally rectify some of the issues that the reformers had objected to and to resist the evil that had been plaguing it. Just my :twocents:
With regard to the early church, indeed some protestant bodies do see themselves as returning to what they view is more like the early church. Some like the Anglicans and arguably the Lutherans view themselves as a continuation of the church that had existed up until the break with Rome simply outside of Rome’s control and still see themselves as part of the one, holy, catholic and apostolic church. Indeed as an Episcopal I still profess that ever Sunday using the more or less the same Nicene Creed as Catholic, Lutherans, Orthodox (minus the filioque).