Instead of using the term schism, the term sect is used by multiple religions and conveys the same purpose, though can also be described as belonging to different philosophical or political groups. Ecumenical Council could just be "Religious World Congress"This would work quite well within the current constraints of Civ VI, where coreligionist states could determine differences in belief through ecumenical councils. There could be many ways to implement this. For instance, we could revive the Apostolic Palace from Civ IV so all coreligionist states would regularly convene to vote on beliefs. I am also open to another of Zaarin's ideas that adopting a Theocracy could trigger a schism on its own, modeling the seizure of religious authority. I will point out, though, many terms including "schism," "ecumenical council," and "reformation" are specific to Christianity and thus not necessarily applicable to other contexts.
