Jihad!

Circassia is Orthodox...

The only readily findable ruler, from the 1460s, was Orthodox, but there was a divide between Muslims and Christians (and traditional 'pagan' religion) with the population being predominantly Muslim after a big conversion boom in the 17th century (which in game is nearing its end) so,

1. It's reasonable to assume that the majority of Circassians are Muslim by this point, with significant Orthodox minorities and perhaps a decent amount of syncretic mixes with traditional pagan influences.
2. I have no idea who the leader of Circassia is at this time, or if there was even a united Circassian polity beyond a loose confederation of tribes. In all likelihood, there are many competing leaders of both Christian and Muslim persuasions.

2 is irrelevant for my purposes though, as I am trying to entice local strongmen and religious zealots into crossing the border and joining my war. If there is a central Orthodox monarch, he is unlikely to have the power to control the movements of his people.

EDIT: On the traditional pagan religion, it is apparently still practiced by up to 12% of some modern day Circassian communities, and seems to be a monotheistic religion with highly varied influences, mostly from Christianity and Islam, but the pagan basis of the religion is very old, despite getting caught up in the whole neo-pagan craze and growing after the fall of the Soviet Union and rise of modern Circassian nationalism.