Suggesting the latest PoD so far:
Let the Papacy crumble completely in the age of the Reformation!
The following potential reasons come to my mind:
- worse economic and military situation of the Papal State itself,
- (some) spread of the ideas of the Reformation into Italy,
- no such staunch, willing, and powerful defender of the Church as Charles V. IOTL.
And I am envisioning the following consequences:
- lacking a clear leader on the Catholic side, no clear alliance behind the traditional creed builds - most princes take a neutral/observant/tolerant position (IOTL some did, like Bavaria),
- princes backing the traditional belief obtain not even support in words from the Pope - because of timidity in the face of Protestants at the gate, or because of other business (a demanding war). Perhaps he even does the Catholic princes some harm, say, by power interests in combination with the war (all of this seems plausible given that the Pope IOTL did not understand the momentousness of the events at all),
- optionally: a dethronement of the Pope - either a (short) Protestant revolution/invasion in Rome, or any other period of sede vacante - not very long but too long in such important times;
- in any case, even the princes most loyal to the traditional lore of the Church cannot refer to the Pope as there is none, or there is one impairing their position like a bull in a china shop,
- by lack of a clear leader (Pope, Emperor, or King), each *Catholic prince has to cope with the new challenge on their own. Of course, alliances are possible when it comes to upheavals or military conflicts, but there is no central authority in religious matters.
- What is valid for *Catholic princes also applies to bishops and abbots who will have to face these challenges as well.
- As the threat to *Protestants from "the other side" is not monolithic as IOTL, there is a lot less reason to ally, and there will also be significant centrifugal powers in early *Protestantism.
- When the dust has settled, we will see a wide variety of different *Christian creeds, but all (or most) of them easily recognizable from OTL's Christianity.
This may look unusual, but I do think that this is a very plausible scenario. At first, while many historical "key" events are, at second glace, just symptoms of much larger processes and despite the Reformation being such an event that would definitely happen some way or other, a lot depends on the detail choices of few people - most prominently, Luther and Charles V.
Moreover, it took many decades for the Catholic Church to reorganize IOTL, until it could actually provide some counterbalance both on an intellectual level and in a, um, PR sense (aka known as Counter-Reformation).
Finally, even as of 1618, the individual Catholic bishops/bishoprics were a lot more autonomous than they have been in the last one and a half century. IOTL, there were even Cathedral chapters of mixed creed, who would take turns in electing Catholic and Protestant bishops. This was only stopped by military force in the TYW. In this ATL, things could become even more chaotic.
Centrifugal powers will be stronger in this scenario than IOTL, but the, um, convex hull of the theological systems among Christian creeds will not be extremely larger (or away) from those existent IOTL. It might be interesting that in some cases, theologians might cherish the memory of the Popes of yore, as well as the principle of Papacy - without recognizing the one or those who claim that position at their time. For some, "Pope" may become an honorable role that, alas, is no longer filled - like the Apostles'.