If you read about the Mazdacs, they are utterly insane but in a weird way they actually are tolerant and quite good at coexisting with other religions. Similarly to the Fulmen Nati they have no problem woth other gods. I vaguely mentioned that the Mazdac even affirms the existencd of some other gods, so Ternion could be a part of it.

Maybe your religion could exist in a sort of limbo, partially incorporated by the Mazdac to varying degrees? Some places might reject the Ternion totally, some might see it as a cult, some as a minor pantheon, some as nearing the status of Selijuk, etc.