This has to be the most hilarious thing I have seen in some time. I created an anti-pope and installed him as Pope. I then make him my court chaplin. Now... my Pope/court chaplin is telling me about this new religion called Catholicism. :rofl:
http://steamcommunity.com/profiles/76561198089571615/screenshot/79128775601424396
http://steamcommunity.com/profiles/76561198089571615/screenshot/79128775601428471
This happens a fair bit as the Norse after you reform the Falkirate, usually because your Lord Spiritual, the Seer, is researching Cultural technology.
I'm not sure if it has to do with heads of religion being unable to convert.