If it were demonstrated that a person whom the Church has declared a saint was not in fact an historical figure, would that show that the Catholic Church is fallible? Put another way, is the historicity of particular saints an area in which the Church claims infallibility?
You have several questions here, all bundled into one.
How would this person have been “declared a saint”? Most people in Catholic history who’ve been declared saints have been declared such by either the ordinary traditions of local people and chronicles, or by local bishops. Infallibility only comes into play if the person were to have been declared a saint by the Pope, in such a manner as to invoke infallibility. (Which is basically the modern canonization formula.)
How do you prove that someone historically didn’t exist, without time travel? You can have a real historical person who attracts unhistorical stories; you can have a real historical person who attracts historical stories from the wrong era. You can have historical people whose historical achievements are recorded under the names of other people or the wrong dates, or under various nicknames that later generations think are other people with the wrong dates.
Heck, you can have historical people who are wrongly declared to be mythical and fictional by historians who don’t know what they’re talking about. This is particularly common in cases where the person is talked about in vernacular or popular literature of the day, because many historians don’t trust history that isn’t written by historians from backgrounds similar to their own, in a format similar to their own, and preferably with footnotes. (Although others don’t trust anything that isn’t a receipt or an inventory, and figure that even historical literature and primary sources are probably lies.) It’s very embarrassing when mythical kings turn out to have coinage and buildings and receipts (and in the case of one “mythical” Mideastern king, a museum of even older historical stuff that he’d collected about the even more mythical civilizations before his day).
Who was that?
Glad you asked, so I could get off my duff and Google my remembrance…
“Ur of the Chaldees” was at one time regarded as being a mythical city, seeing as it appeared in the Bible as Abraham’s hometown. But after a while, historians and archeologists found other references and realized it was a real town and one of the oldest cities in the ancient world. So they went looking for it, and there it was, right where it was supposed to be if you had the right texts.
Back in the 1920’s, Leonard Woolley excavated some of the city of Ur, including the palace of King Nabonidas, who restored the ziggurat of Ur, and was once just a name on “mythical” king lists. And there they found a little archeology museum founded by his daughter Ennigaldi, including all sorts of bits of earlier civilizations and a clay tablet museum label. A 2500 year old museum, and full of stuff a couple thousand years older than that.
Might I offer a potential example to be discussed?
Andreas Oxner was beatified in 1755. Do you believe that he was captured and killed by malevolent Jews who sought to use his blood in their Passover bread? Or is this more likely to be an utter myth, produced by the rabid anti-semitism of the time? In which case, it seems the Catholic Church erred in claiming him to be a martyr.
“Person” and “historical figure” as a prerequisite to canonization or declaration of sainthood but tradition (there are lots of saints who have been declared saints before the canonization process) could be problematic…if by person, you mean human, it’s not a requirement (st Michael the archangel is not human)…and if by historical figure you mean famous or well known, we’ll more saint were obscure before sainthood than were famous or well known.
Peace and all good!