I have been in a Greek Orthodox church only once. (It was in Greece so it's official.) The layout was different from the Catholic tradition in which I was raised. There was a screen or small wall dividing the front and rear of the sanctuary. We were told that it marked a symbolic separation of the seen and the unseen: some things are properly visible to living physical beings; others should remain cloaked in mystery, knowable only indirectly. I've wondered how far this principle extends.