uhm isn't duke is highest noble position, it's only a level lower than a king, so how the hell this saint.? can banish him.
depend on type of government, laws and culture. Religion and state are inseparable in the distant past.
Closest example is when a pope excommunicate a ruling nobles, royals and/or heirs. It is a big deal back especially before the reformation. Another example is the British law "Act of Settlement" that forbid Roman Catholic from holding the crown and have to maintain the Church of England and the Church of Scotland.
Even outside Europe this is the case in varying degree.
The mc here opposed the state god's authority and there might be some law akin to the "Act of Settlement" where the nobles have to adhere to the gods and maintain the faith .