Who knows... It's hard to say much anything about such ancient past.

It's got zilch to do with religion, but...

Someone whom the masses respect will have an easier time getting them to listen. Humans and youkai both.

Although, I suppose you're one of the people who can attest to its effectiveness?

The rest comes down to where Byakuren takes them from there. This... dynamic missionary work is probably best for Gensokyo.