Why is a doctrine true because a lot of early Christians believed it? Before I became Catholic, but was a Protestant looking in and seeing that early Christians believed a lot of these Catholic things, I had this general idea that these things were revealed directly by God. It was good enough for me to convert (that and finding that EENS and that I was vincibly ignorant), so I did. Now, though, I don’t quite get how you have these beliefs that can’t be proved directly by Scripture or observation such as purgatory (which makes perfect logical sense, and I seek indulgences often). What I want to know is when the belief in (let’s use purgatory for a concrete example) crops up and where the belief comes from (inspiration by God?). For the particular instance of purgatory: I’m not interested in knowing that pre-Christ Jews believed it (cf Maccabbees); I’m interested in knowing why they believed it. Is that just lost to history? For the sake of defense (since a Protestant interloper will likely reject these), let’s disregard private revelations and apparitions.
This is two questions:
How is sensus fidelium infallible?
How does a particular infallible belief that can’t be proven by experiment or not given directly by public revelation come to be?