Here's my wild, most likely wrong but just for fun, theory (for readers of this thread but not Zahariel's other stories, SPOILERS for Roboutian Heresy follows): Cain's possible/alternate future as Living Saint reaches back in the timeline to obscure current Cain's past/future, due to the timey wimey nature of the Warp.
Actually, have an omake about that:
Castellan-Commissar Cain, after that fiasco at Cadia, was not surprised that the Warp was out to get him. Thus, he was appalled, but not surprised, when even with the Emperor's Light suffusing him, he was swept into a Warp rift when the Void Shields on his flagship temporarily failed, and he was flung into an alternate past where somehow the local Ciaphas Cain was a Chaos Cults Leader of a single planet. And the 'visiting' Cain was in all his Golden glory of a Living Saint, his presence in the warp was such that the local Emperor could have felt him on His Golden Throne.
"... Well, frak," both Cain chorused as they stare at each other in horror. Both of them came to the conclusion that the Emperor and the Chaos 'Gods' have shit taste in humour, as Castellan Cain could still feel local Cain's faith in the Emperor, buried under all the excuses and self-preservation - just like his own, but then again they were both Ciaphas Cain. Not even a miracle could have explained how local Cain wasn't eviscerated by Chaos forces, so Cain concluded it must be deliberate intervention by the Great Enemy.
"Leave us," local Cain ordered his underlings. "This is not going to end up as a fight, and even if it does I don't want you all to throw away your lives for nothing."
"... As you command, Lord Liberator," the minions said grudgingly as they withdrew from Cain's office, including the local Jurgen whose paradoxically-clean face brought painful memories up for the visiting Cain.
After a pregnant pause, as both Cains waited for others to move a distance away, Living Saint Cain raised a barrier around the office so they can talk without being overheard, barring maybe by the Emperor if he is listening, since Cain's powers came from the Emperor's own. Working his mouth, the Cain in Gold began:
"... My condolences," Cain said sympathetically.
"So does that mean I won't end up as you, that my redemption is set in stone and the Emperor will welcome me back?" Younger Cain sighed, all posturing draining away now that there was no one there he needed to pretend for.
"Who knows," Older Cain said. "I don't, probably not even the Emperor does. All I am saying is I feel for what you will have to face in your future, you have a lot ahead of you. I myself went through over a century of craziness before now."
"So would I end up as a crazy Chaos cultist, destined becoming a daemon to topple the Imperium, a mockery to who I am now?"
"Only if you give up. Only if you choose to fall to Chaos."
"... I hate this."
"Yes. You'll probably end up having to deal with more trouble as 'Liberator' than I did as 'Hero of the Imperium'. At least all I had to worry about every day was death, and not damnation except on occasions that's uncommon but still not rare enough to be comforting. This Sainthood was a very recent thing."
"Do you have any more advice to give your younger self?"
"While we are the same man, our background are vastly different. In this timeline Horus was the Primarch that Chaos propped up, but it was a different Primarch in mine. Whatever advice I can give you will only be generalities as opposed to Events that you can manipulate for your benefit. I believe the Eldar Farseers suffer enough from that mistake that you wouldn't want to repeat for yourself."
The visiting Cain only managed to spend a few minutes giving whatever paltry wisdom he had, before he felt a furious daemonette trying to knock down his barrier.
"Frak, that was a fast response."
"... Not really, that's my, well, girlfriend. She's a local. The others must have told her what's happening."
"... If you do not prefer blondes, I see that we are not the same man after all."
"I live in fear every day that she might discover me for who I am and eat my soul! I didn't ask for this!"
"And I thought having a Inquistor as a lover was perhaps bad for my health."
"Wait, blonde woman, Inquistor... Your lover was Amberly Vail?"
"You know her?"
"We've met. She got away safely."
"... My thanks."
"So, how are we going to play this?"
"I'll defer to your expertise in herding Chaotic felids."
"I hope your acting is better than mine after a hundred years' experience, old man."
"Hah! You're better off praying to the Emperor, youngling, because it never gets easier!"
-][-
What followed was supposed to be some kind of epic showdown between the a Favoured Servant of True Gods and the puppet of the Corpse Emperor, complete with banter and wanton violence, with the soul of the Liberator on the line, but the Living Saint merely casually backhanded Emeli away before teleporting to destination unknown. Cain the Liberator had to appease his daemonic patron that he was unharmed, and to reaffirm he has turned away from the false God. Which he had done with vigorous rites with all of the Handmaidens for six days and six nights, before he had to step in to save Slawkenberg from its next crisis.
Truly, how horrifying.
-][-
The Living Saint Ciaphas Cain didn't manage to return back to his home timeline before he was plucked from the Warp by the Emperor Himself, brought to Terra and start a Crusade, before he was allowed to leave. Meeting the Emperor in person didn't change Cain's impression that the Emperor's humour sucked, even if he can empathize on what needed to be done with all resources He had available to him.
At least 'Ciaphas Cain, Warmaster of Chaos' didn't get as much trouble as he could have from the Imperium, now that it's revealed he can be a true asset to the Emperor- no, Cain couldn't finish that line of thought and pretend it was true. There are going to be many wanting to remove a possible Living Saint joining Chaos, and many wanting to 'rescue' Cain from Slawkenberg so he fulfill his true potential. At least it wasn't this Ciaphas Cain's problem now.