When Batman was sent back to the beginning of time, he inadvertently set in motion a series of events that would lead to inspiring himself to later become Batman. Wearing the carcass of a giant demon bat, he inspired the ancient Miagani cave tribe to worship him and await his return for thousands of years. It was his actions that inserted the inherent fear of bats into all men.
It is the crumbling relic of his cape, found by Red Robin in a secret area of the Batcave that proves to the world that he is only time-lost, not dead. Now, having returned from the grave, and having become a defining force in mankind’s development, Dick Grayson tongue-in-cheekly, but tellingly introduces him as BatGod on his return.