You write him a letter in which you want to make it a proper deal in public and with proper level of cooperation between the two armies, rather than making it a shallow secret. You do not mention anywhere you want to give up on this proposal.
875 AD, Late Winter
Thanks to the squeaky blaring of the 57 goat-trumpets of the Pantheon, you learn of the Archpriest's arrival before the guards even reach your throne room.
The Archpriest, as always wearing thick white robes, bows his head as he enters the council room and sits on the opposite side of the table from you. The servants bring some snacks and drinks and then the Archpriest dismisses his entourage; you dismiss the guards and servants of the palace.
"So, what brings you here, Archpriest." The middle-aged man smiles.
"It is hard to call it heresy, but you have been making inroads into workings of the Cult, King Feodh. What is it I heard, sponsoring the scribes, allowing construction of temples of rememberance?"
A) "That is not your concern, Archpriest. I don't pry into your administrative decisions, and you shouldn't into mine."
B) "I am merely responding positively to the audiences that my subjects wish for. If they want to build temples to honor my ancestors, I shall allow them; if they want to write biographies praising us, I will be most happy to let them."
C) "Make love with your arse and don't waste my time anymore, Archpriest! Luathbas and its subjects belong to Houlandines, not the Pantheon!"