My character, Brance Indirik, having been present at the founding of the religion, vehemently disputes this assertion. He has argued constantly against the Holy Prophet's infallibility for IG years.
As a player, my biggest disappointment with SA has been with the strong Catholic influence in Sanguis Astroism. There was a trend within SA at some to for all the priests and elders to call other people "son" or "daughter", and to refer to priests as "father". We had positions named "cardinal" and "bishop". And once someone started doing it, others quickly fell in line and started doing it, too. I am fairly certain that the infallibility trend started along the same lines as Papal Infallibility.
I found it extremely irritating. In fact, one of my biggest personal goals during the time I was regent was to revamp the rank structure to get rid of stuff like that.
Indeed, I remember the old infallibility debate with Brance and Allison; finished inconclusively but Constantine is still a firm believer.
Although I agree there has been some Catholic/SA crossovers in places I'm don't think infallibility is one of them. Essentially the Pope's infallibility in Catholicism is very much based on whether the doctrine declared was already accepted (he can't make new stuff up only reaffirm old stuff that is not yet dogma) and he can use his infallibility only when teaching "ex cathedra".
The Prophetic infallibility held by some in SA (including Constantine) is much more powerful; essentially comparable to the status of the OT Prophets in Christianity and Judaism and the Prophet Mohammed in Islam; a "words coming directly from the divine" kind of thing that the Papacy never enjoyed.
Of course I can't speak for the other players but I can say for certain that Constantine's views on Infallibility weren't drawn on any perceived relations with Catholicism but rather from his understanding of the nature of Prophet-hood as well as his personal encounters with Mathurin.
In any case I think Mathurin has done a great job of ensuring that there isn't a schism on the matter