Eamonn Clark, STL
Most sedevacantists claim that John XXIII was not validly elected pope because he held something heretical prior to the conclave, thus he was not a Catholic, thus he was ineligible for election.
While I think that is incorrect, here I will grant it for the sake of argument.
Here’s the question: how do you know that was the first time? Don’t you think it’s possible that, say, a weirdo like John XII was not totally orthodox prior to his supposed papacy? What about even earlier, when we have practically no record of anything about the early lives of the supposed popes? Doesn’t that disturb you? Wouldn’t it be more likely that God doesn’t ever let His Church fall into such a state where the vast majority “goes along with it” and doesn’t really worry about it?
Just asking.
Christian Renaissance Movement 
This is a natural extension of uncharitable scrupulosity of the priestly class by the laity. The funny thing about this scenario is that John XXIII was elected by a papal conclave, which is ostensibly a process steeped in the workings of the Holy Spirit. That should short circuit all of this thinking: “Hey, John XXIII had some weird things he said back in the day but the Conclave picked him so he must be fine now”. The laity think they are the stewards of the Church–which is true but not in this specific sense. The laity are stewards of the Home Church, of their own and each others praxis. They can scream and shout about gross liturgical abuses, but splitting hairs over past pronouncements and declaring unilaterally (!!!) that a papal conclave was invalid–that crosses over the line.
I did not know there were sedevacantists who date the deviation back to John XXIII. I’m a relatively new Catholic (2018) so perhaps my ignorance can be forgiven–I am sure there are some who are not happy with every pope ever elected and this sort of drama will be unending.
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