From the kickstarter page:
Backers at the Cultist level and above will also receive the special pack of promotional Cult cards that grant even more powers from your Ancient One!
These cards can be optionally added to any game. There is a Cult card for every god in the base game, as well as Cult cards for Azathoth and any of the gods that are unlocked through stretch goals. There are even a couple of unaligned Cult cards for even more variety! This promotional pack will not be available in retail and is a little something special to say thanks for supporting the Kickstarter.
Or even better: Cult Cards (Promotional)
(Edited to add thread link with even more details)