I'm just starting Bioshock Infinite and came across a building for the Order of the Raven. I interrupted their meeting and killed all the guys assembled near the altar, and on it I found a key. When I picked it up, it said it was for an optional quest, and that I should go back to where I've already been to find somewhere to use it.
I ran around a bit through the area I'd already explored, but didn't find anything to use the key on. Where can I use it?