"Church key" is slang for bottle opener. The large type of bottle opener that has the circular bottle opener thing on one end and on the other end the pointed can opener (the kind they used to use to open beer cans when beer cans were made of steel and didn't have pop tops, the kind that makes a triangular hole in the top of the can), this kind in particular, resembles the large keys carried by certain members of the clergy. The term "church key" has come to be used to refer to pretty much any kind of bottle opener now though.