But still, what actually is "rigorism"?
My best guess: Rigorism is the rejection of the decisions made by a given church's hierarchs in favour of an apparently or actually more conservative option (a lot of these groups have "old" or "ancient" in their names), manifesting as contempt towards the hierarchs in question or outright splitting from them. I'm not sure if this is an essential element, but they usually revolve around issues of practice that seem laughably trivial to outsiders; you'll see calendars pop up quite a bit here. Examples, with the issue they split over in brackets, include:
The Essenes in Second Temple Judaism (asceticism, the use of the Enochic calendar and the legitimacy of the Second Temple)
The Donatists and Novatians in Patristic Christianity (readmission of repentant apostates)
Old Calendarists in Greek Orthodoxy (the Gregorian or Julian year for the calendar)
Old Believers in Russian Orthodoxy (icon styles and how to make the sign of the cross)
The Ancient Church of the East in the Church of the East (the Gregorian or Julian year for the calendar)
The Old Catholic Church and Sedevacantism in Roman Catholicism (papal infallibility and the liturgical reforms of Vatican II respectively)
The Fundamentalist Church of Jesus Christ of Latter-Day Saints in Mormonism (polygamy and the Adam-God doctrine)
Having made this list, I think there's an additional crucial trait unifying them: they all come from groups that in some fashion or another claim to be the True Church, and that's what gives them their unique character - they insist that they, a small schismatic group, are the only true Christians.
