I can appreciate that "canon" is used in religious contexts, but its meaning and origin go back further than that. Wiktionary (hardly the best source, I know) lists three secular meanings before the religious ones begin. The most relevant is this one:

> A group of literary works that are generally accepted as representing a field.

Also, the coinage "fanon" (as in derivative works by a fanbase) comes directly from the treatment of the source material as "canonical". (Frankly, "canonical" is the best argument I can make for "canon".)