Also, since he is a Cleric, and since True Faith and Psyker abilities are mutually exclusive, you could use these instances as a way to have the Cleric begin to discover True Faith talents, and so forth. However, the potential reason this might not work would be how you imagine the experiences to have affected the character. Is it possible that these experiences increased the zealotry and conviction of the character? The Emperor is supposed to have a presence in the warpâ€¦did the character actually see the Emperor's warp presence? That would be a good reason to become inspired by the Emperor. And characters do gain insanity as a result of exposure to Imperial influences sometimes. On the other hand, if it was more of a deamon's-corrupting-power type of exposure, perhaps the Forsaken Priest, page 42 of the Radical's Handbook is the way to go, with the varied Arcana and Sorcerer advances. If that is how you want to do it, I would start off by giving the F.P. the Sorcerer advance for free, and giving him automatic access to the FP alternate level whenever he chooses.
I have taken extreme exception to your point of True Faith and Psyker abilities being mutually exclusive. Where in all the rulebooks, have you found a rule by RAW that so states?
Extraordinarily rare for them to exist side by side… I might agree with. But mutually exclusive? That would lead to the suggestion that the God-Emperor, who is considered by many to be the most powerful Psyker ever, to be a Heretic.
Also, such a character would be… powerful in the extreme, if not kept in check by some wise GM.
For example, I have an NPC in my game that does have both Psy powers and Pure Faith. Her extremely limited Faith powers are more of the 'always on' variety (Aura of Faith is one of them, and that she elite advanced to take)… things that she's not even consciously aware that she has.
As far as rulebook allowing this, you should check the Ascension handbook and the class of Inquisitor that can have both Pure Faith and Psy powers side by side.
"It's what we do... we live, fight or die, at the God-Emperor's graces"
... Inquisitor Mardenicvs Aneas Rexvs, shortly before his death.