Let’s start with the core mechanic of the Nahua Ritualism, an old EU4 friend, Doom:

As you can see, Doom accumulates over time, and the bigger the country is, the more Doom it accumulates. It can be mitigated either by performing some mechanics, such as killing enemies and looting locations, or by some of the Religious Actions. But there’s only one way of completely escaping from it, which is Reforming the religion. This can be achieved by passing by enough Religious Focuses, the former EU4 ‘Reforms’: