I started in 1018 as the guy ruling that duchy in the Middle East with Eastern Syriac and Suryahe provinces. I switched to that culture and religion, and after I had significantly expanded a heresy appeared: Messalian. It seemed interesting, so I switched to it and found that it is far too powerful. Once the religion is large enough, its moral authority is constantly at 100 due to something called "self-castration" which stacks too much, as shown in the image. Also, the head of the religion was my vassal and I always made him my court chaplain, ensuring high relations with him and fast conversions. Messalians are still allowed to join the Eastern Syriac society, which means they have access to all those bonuses and the ability to instantly convert any province once the highest rank of the society is reached. Messalians have access to the "Driving out Demons" decision, which removes negative traits and adds positive ones. Almost any Messalian character can take a decision to become a "Perfecti", which grants some learning and piety and +20 opinion with *all* Messalian characters. This opinion bonus is huge and makes rebellions and assassinations less likely. This means that a Messalian king is able to have high relations with most of his vassals, remove negative traits, and quickly convert characters and provinces.
In the religion map you can see that I spread the religion extensively. Due to the permanent 100 moral authority, provinces very rarely deconverted once they became Messalian. I invited pretenders to my court, converted them, and installed them. The collapse of empires needs to be clearer, I think. The Byzantine Empire had been stuck at 100 imperial decay for a century, yet it never collapsed. Conversely, the HRE collapsed after being at 100 decay for about a decade. Bordergore is still a problem, and the rules for breaking realms upon rulers' death need to be harsher. I was using the harshest settings, and there were still oddities like the Fatimids controlling northern Ireland.
In the religion map you can see that I spread the religion extensively. Due to the permanent 100 moral authority, provinces very rarely deconverted once they became Messalian. I invited pretenders to my court, converted them, and installed them. The collapse of empires needs to be clearer, I think. The Byzantine Empire had been stuck at 100 imperial decay for a century, yet it never collapsed. Conversely, the HRE collapsed after being at 100 decay for about a decade. Bordergore is still a problem, and the rules for breaking realms upon rulers' death need to be harsher. I was using the harshest settings, and there were still oddities like the Fatimids controlling northern Ireland.