The End Temple is the capitol of the Church of the End. This Religion worships the Endermen and uses the Ender Pearl as it's symbol. It is the most neutral of the three religions, with the Church of Man and the Nether Religion (name?) in a constant state of war. The end temple is one of the ways to enter the End, and is a one way trip due to the Dragon devouring the sacrifices dropped into the End Portal.
For years, the followers of the Church of the End have dropped people in as a sacrifice to the Ender Dragon to keep him from attacking the Overworld.
At the moment, the End Temple is a quarry, still being filled with black wool on the bottom and edges. The upcoming End Temple will be built above.