It's not as wild as it sounds, and it's not just because 'ficcers chose "canon" to define what the true source material is.
The Nicene Creed is Christianity's FanLib.
A lot of energy is expended nitpicking the source material, fanwriting (let's nominate Orson Scott Card for "Best Fanwriter" next year!) and fighting over who the most dedicated fan is.
The Pope, James Dobson and Jerry Fallwell (and for that matter all the Elders, Imams, Ayatollahs and the rest; it's not just Christian sects that map this way) are BNFs.
Interdenominational squabbles are usually about which BNF is bigger, or about what fanon is relevant and irrelevant.
Megachurches and traveling big-dollar arena revival shows give a new ironic twist to "Creation Entertainment."
And, of course, there are a lot of consumer-fans.