Yeah, Christians and Jews really hate the Dalai Lama.
But, in all seriousness, looking at the Mughals and other Indian states suggests that (or the Manichees or Nestorians earlier), in at least some cases, cross-religious spiritual authority of some kind was recognized.
Again, for a Medieval thinker, spiritual discussion is not relativistic as it is for us. We believe, or most of us do, that the most direct method to objectively debatable reality is probably the scientific method. Medievals, mostly (a few interesting dissenters among Christian heretics and some Muslim philosophers come to mind as counter-examples) thought that spiritual authority was a direct and objective thing. It's like if two scientists with radically different approaches to, say, evolution, just said, "Well, your model can be right for you, mine is right for me; ultimately, who's to say?" No, those people have a common idea of a methodology for debate and argument.
Similar things existed for Medieval philosophy and theology; common understandings of debate (such as the need for classical precedent in much of the Christian and Muslim intellectual world). They wouldn't say, "Don't bring your religion in here!" they would say, "Your religion is false, for we all know that Plato has said XYZ, which you erroneously interpret..." Now, of course, we don't have Plato. But the general idea should remain: that our characters, being Medieval, will not regard religion as a purely subjective matter, or something without recourse to objective mediation.
And I have often wondered why Mathurin doesn't take the Dean of Theology position.
You're missing my point. In your fictional example, both religions are based on the teachings of Plato, and the two different religions can debate productively the meaning of his words and how they should be applied. The same is true of the Abrahamic faiths and even more true of denominations under the Christian umbrella - they all purport to worship the same deity and accept a common mythology.
An Abrahamic faith does not have the same relationship with Hinduism. There's very little common ground on which they can meet to debate something on the merits. Religions in BM are the same way. If we ever have a schism in SA, there would be much more fertile ground for this in the divergence of one faith into two or more, all debating the true nature of the Stars and the value and meaning of Mathurin's teachings.
