What's your favorite Christian heresy?

[World politics is up Schmitt creek]

Collyridianism if it actually existed, Patripassianism if it didn't. I also have a favorable view of most of the movements PSOL mentions, with the exception of Gnosticism, a school of thought of which people have a ludicrously high opinion considering its unabashed secrecy and elitism--probably since, unlike with a lot of other heresies, enough Gnostic texts survive that they're more or less able to speak for themselves.

Thomas Wycliffe and the Diggers

John Wycliffe was with the Lollards, not the Diggers, whose most familiar individual figure is Gerrard Winstanley.

[World politics is up Schmitt creek]

     I like the Old Believers and use several of their cosmetic practices (e.g. using two fingers to make the sign of the cross), though the anathema on them was lifted many years ago. Some of the sub-sects of the Old Believers had very unusual doctrines that I find fascinating.

     It is hard to pick just one group to call out, but I would like to mention the Holy Thursday Gapers, who would keep their mouths open during liturgy on Great and Holy Thursday in anticipation of receiving the Eucharist from angelic ministers. Their reasoning for this was that they did not believe any valid human priests still existed on Earth, so Holy Communion could only be given to them by angelic hands.

Were the Nikonian Reforms really successfully enforced on every Russian Orthodox priest, or did the rationale for this have to do with something else, like a lack of "faithful" bishops to perform new ordinations?

[World politics is up Schmitt creek]

     I like the Old Believers and use several of their cosmetic practices (e.g. using two fingers to make the sign of the cross), though the anathema on them was lifted many years ago. Some of the sub-sects of the Old Believers had very unusual doctrines that I find fascinating.

     It is hard to pick just one group to call out, but I would like to mention the Holy Thursday Gapers, who would keep their mouths open during liturgy on Great and Holy Thursday in anticipation of receiving the Eucharist from angelic ministers. Their reasoning for this was that they did not believe any valid human priests still existed on Earth, so Holy Communion could only be given to them by angelic hands.

Were the Nikonian Reforms really successfully enforced on every Russian Orthodox priest, or did the rationale for this have to do with something else, like a lack of "faithful" bishops to perform new ordinations?

     Indeed it was a matter of bishops. They had priests initially, but no bishops of the Russian Church were with the Old Believers to ordain new ones and so their priesthood rapidly died out. Their most prominent members were able to convince a deposed bishop from another country to come and ordain new ones, but these ordinations were not universally recognized among Old Believers. This led to a division between the "popovtzy" who had priests and the "bezpopovtzy" who did not have priests. Most of the truly bizarre sects of the Old Believers were bezpopovtzy.

On the merits, I'm actually sympathetic to the contemporary arguments against the Nikonian Reforms. Everything I've read about them indicates that they were imposed in an incredibly heavy-handed way, and the rationale for them reeks of cultural cringe. I probably would have grudgingly accepted them once they were in place, though.

[World politics is up Schmitt creek]

Sedevacantism does have legitimate grievances with Rome.

I don't mean this question as an attack or even a criticism really, but why exactly would a staunch Reformed Protestant like yourself understand internal Catholic Church issues well enough to hold this position, especially since a huge sede issue is perceived "Protestantization" of things like the liturgy and Biblical theology? There's way more to the rationale behind sedevacantism and the Traditionalist Catholic movement/subculture in general than just "Left-Leaning Pope Bad", even if "Left-Leaning Pope Bad" currently eats up a lot of many trads' intellectual and emotional energy.