I seem to be encountering a bug with my game. I do not have any mods and I uninstalled the BG2fixpack. (I did end up finding a fix, see below if any else has the same issue.) I am a Lawful Evil cleric/fighter. I have not been Nalia's keep yet and entered the temple district, saw Gaal' speech and then was approached by Osig after. I accepted his quest because at the time all the work arounds and bug fixes didn't work for me. Also hoping that afterwards I could just talk to the Temple of Talos and everything would be fine.
I did not go and talk to Osig after completing the quest, instead I went to the Temple of Talos and used the consol to create Vilon. He talked to me just fine and then my journal updated with a note saying Cleric Stronghold and that I was now in the church of Talos. However, when the first quest to talk to showed up, there was only "NO VALID RESPONSES" present. Clicking that exited and I couldn't reiniate dialoge with him.
So I the reloaded my game, went into the church and created Stormherald Nallabir using C:CreateCreature("bhnallabir") . He then proceeds to ask if I killed the beholder (just like Osig did after I talked to him previously to see what would happen) I say yes, then got my XP bonus, a journal entry saying Cleric Stronghold, and then Vilon showed up. Talked to the first quest guy and dialog appeared normal. Vilon didn't show up immediately afterwards, but I will give it a few days game time and see how it goes.
So this seems to fix a bug where you get approached by the wrong church, but you may be able to complete the quest then spawn in the clergy of the church you want.