That the Fire Magician was used (unwilingly) as part of a summoning Ritual when they were in aprinticeship. Later a Forget spell by the master removes it from the PC's knowlage.
The puropse for including the young aprentice was incase of Ritual/Spell failure. If the master is linked with someone when a spell goes bad, they can try to channel the failure to whoever is in their "link" so they survive. Obvously the master was trying something new and powerful to need a backup.
The end result of said magic created a link between the PC and the Demon. Thus everytime the PC calls forth the Demons element (fire) there is a chance of influence or bleed through.
I would suggest that after each such event the PC has strange dreams. Either trying to teach hte PC specific summoning teniques or trying to lead the PC to acomplish the Demons desires. If the Demon is real smart, he will present it to the PC as if the Demon is a favored spirit or small gawd of somekind. The PC may never know they are working for evil.
Unless someond uses Detect Evil during one of these dreams or there is enough residue to be picked up on by other PC's or NPC's.
Most Wizards dont go into churches on a regular basis anyway.