More guesswork ... (some context would be useful).
Another possibility - "authoritative agent"? .. who is perhaps someone with the authority or who has been given the authority or (more importantly perhaps) is considered an authority - an expert - maybe unquestionably so (who can interpret the holy bible, scriptures etc... or who can make decisions or can take charge of something in general on behalf of others, owing to his expertise and special knowledge? - I don't fully grasp this term, and I'm guessing here).
The term "authoritative agent" might even apply to an entity - like the Catholic church, an advisory body, a council etc..? -