Attitudes towards foreign Immortals
by HåvardFor the Church of Thyatis, I have always been of the opinion that there is a bit more to it than just a wide bunch of cults devoted to individual immortals. There is for instance an overarching Faith connecting these cults, even though the organisation connecting them is very lose at best.
Syncretism exists within the Church of Thyatis, but much less so with the Churches of Karameikos and Traladara. (Darokin is open to speculation).
The Antalian Faith, if we go with the RW Aasatru parallel, accepts the existence of other Immortals, and acceptance of healing/aid and even usage of relics from other immortals, but joining a foreign cult is absolutely banned. In Antalian society, religion is firmly connected with the clan structure, and abandoning one's religion means turning your back on your family and clan.
The Ethengar would probably simply assume that others worship various hero spirits (true, in fact), but probably feel little reason to involve these in their daily lives since they are not connected to their lands or their people.
The Eternal Truth is even more absolutist than the Church of Karameikos, and would not allow worship of any immortal other than Al-Kalim.
The fact that Mystara has clerical magic does indeed change things from the R/W situation. Few people doubt that the Immortals exist, even immortals outside their own Faith. However, the nature of those immortals will be disputed. Most foreign Immortals will be suspected to be untrustworthy at best, demonic at worst. R/W comparisons may be something like Faeries and Demons. Okay, so Faeries aren't exactly evil, but they are likely to get you in trouble no matter how many presents they offer you.
This is not so different from the R/W medieval situation actually. Unlike the situation these days, medieval man rarely disbelieved the existence of magic and supernatural beings, both of benign and malign existence, though he was likely to rely on the local priest to tell him which was which.
The closest to Atheists in the Known World are the people of Glantri. They also do not deny the existence of Immortals, but frown upon Clerics which they see as people who have sold their souls to more powerful beings rather than pursuing the power of magic on their own.