"redeemed" Athair.

I imagine it'd be either treating the defector as a colleague who has "fallen from grace" or going the "never-speak-of-him-again" route, though those Athar who truly hate Power-worshipers might consider roughing up the defector, or even killing him depending on the nature of his change-of-heart (and also what knowledge of the Athar's inner workings he's taking with him as he goes).

How a former Athar can convince his newly found religious leaders that he's sincere depends on the nature of the religion, I'd imagine...

Personally, I like the other option for a defecting Athar, which is to say one that has (ironically) lost his faith:

"The Powers are just supremely powerful berks making a play at true divinity. That which is truly divine exists beyond mortal - probably even immortal - comprehension. That's what I believed. I spent several lifetimes learning, studying, seeking out that elusive spark of the Great Unknown. I had the chance to be Factol, but turned it down to devote myself more fully to my search for Truth.

"I truly believed that I was becoming something more, something greater than the mortal husk I was born as. I pulled away the veils of illusion and falsehood that the Powers and others have layered upon the Multiverse, with each one growing closer to the object of my search.

"I could see it in my mind's eye: the beatific glow of the truly divine, growing brighter and more real as the illusions fell away. I understood so very much: the machinations of the so-called gods transparent to my eyes; the convoluted course of the Blood War like the simplest of board games. I could have done so much, helped so many, but in my fervor to touch the truly divine I left it all behind.

"And then, I saw it: the final veil. The light of the divine was shining through it with such intensity as to blind a lesser man, one whose eyes had not long since adjusted to the light of Truth as mine had. Without hesitation, as the culmination of centuries of work, I parted the veil to look upon the Face of that which could truly be called God.

"There was nothing there."

- Some barmy Bleaker locked up in the Madhouse