So, as much as I'm kinda meh about CK2 these days since on my PC it just runs so sluggish these days that it get's tiresome to play, I do still play it occasionally.
In my current campaign an interesting religious situation happened... basically, except for hold outs in Spain, Britain, and a couple counties in Anatolia due Hospitaller Knight gains in early crusades, Catholicism is dead, even the Papacy is exiled from Rome, in their place is Orthodoxy, Monothelitism, and Lollardy. It all roots back to some Orthodox King taking power in the Empire of Francia and instead of the lords resisting this they all basically all signed up.
Now another interesting Facet of all this is my Dynasty, the Emichids. The Emichids, as far as I know have Five independent realms under their rule, four of which each have their own religion..currently.
My character: King Karl Emichid III of Holy Germania (Founded this kingdom myself actually) which champions the cause of Monothelitism
King Tello Emichid of Leon, which champions Catholicism.
King Augustin Emichid of Middle Francia which champions good old fashioned Orthodoxy. (Though middle Francia is an elective monarchy which is surprisingly quite varied on the Dynasties they get elected.)
Petty King Cenraed Emichid of Mercia which also champions the Monothelitism of his direct Holy Germanian roots.
and finally
Petty King Ealdwulf Emichid of Essex which champions Iconoclasm...
Yeah, House Emichid, is not exactly the most religiously united dynasty ever.