Actually, I believe you need to tell Firkraag you'll be back for him later, and refrain from killing him. Then, when you return to the Radiant Heart building, you will get the quest to go and kill him. Once you do, you will be accepted into the order. This gives you more XP as well. I hope you still have a savegame from before you killed Big Red!
If not, try talking to the prelate, who is in the small room in the North of the Most Noble etc building and Ryan Trawl, who is standing in the great hall in front of the entrance. If neither of them has anything new to say to you, I think you may have lost your chance at completing your stronghold quest
