What would a cult member call a non-believer?

Mundane is fairly likely to see usage this way, referencing someone being concerned with "matters of this world" as opposed to whatever the cult is into. A cult less into traditional dualistic slurs and more into cutesy neologisms might use sheeple. Cults that are about awakening the higher perceptions or psychic powers of their members may derogate non-members as headblind.

And, of course, from A Scanner Darkly, we get the immortal phrase albino shape-shifting lizard bitches, which really cannot be applied frequently enough to describe anybody the speaker wishes to categorize as "other".

How about a nullifidian? Could be a word the cult leader decided to revive.

I propose heretic, apostate or paynim.

Philistine has that appropriate apocalyptic tenor characteristic of cult members:

Just as hath the Lord promised when He delivered those woeful people unto David and his mighty men, so shalt you and your heathen ilk burn in the fiery furnaces of Hell for all eternity!