There was a somewhat brief thread about this a little while ago:
https://www.alternatehistory.com/discussion/showthread.php?t=167217
What I wrote then was:
Good topic! This is one I’ve mulled over every once and while too. A couple of ideas come to mind:
John Paul I was a very quiet and retiring figure who by all accounts was rather intimidated by his new power and responsibilities. Thus I imagine he would likely be a rather hands-off pope. Though he did have some interesting ideas that he felt rather strongly about, so perhaps if he gets himself a more forceful lieutenant who knows the ins and outs of the curia, he could make some waves.*
I imagine that South America would be an area where some interesting differences would occur. Before he became pope, John Paul I had traveled there (one of the few times he ever left Italy) and was good friends with the Brazilian cardinal Lorscheider. And of course his pontificate would see the rise of liberation theology, which would likely not be suppressed quite as forcefully as OTL, so that could have some political repercussions on the wider world.
John Paul I almost certainly wouldn’t travel as much as JP2 did, nor would he probably come up with World Youth Day, etc. He would likely be viewed as a kind and gentle old man, but wouldn’t have anything close to the mass appeal and star power of JP2. I wonder if without such a powerful central figure, the church might grow more decentralized and regionally-focused. No official changes in structure or hierarchy, but in terms of people’s mindset.
Also: Eastern Europe. No Polish pope to add moral support to Solidarity, etc. But on the flip side, I would be very interested in seeing what kind of a role Cardinal Wojtyla plays back in his native land.
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*IIRC, he once suggested that first-world dioceses should take an active role in supporting their third-world counterparts who were hard-pressed for the resources they needed to run effectively, but who knows if such a policy would ever get enacted.