All religions - regardless of any evidence to the contrary - claim that their god is the One True God, more powerful than all the other gods. The religion with the least evidence, however, is the extremist group who worship the God of Gods.
According to their gospel, the God of Gods created all the other Gods, then had a fight with them and then disappeared and no one has seen him since. As such, worshippers of the God of Gods shun all other believers, refusing to interact with them at all. They have their own isolated societies, and friendly conversation with someone outside the church is enough to get one totally ex-communicated.
Unlike other worshippers who are gifted a special power after declaring devotion to their deity*, those who have the God of Gods as their chosen deity experience no visible benefits, though they do claim that "they are just generally better at most things" as a result of their worship.
*such as exceptional lock-picking ability (worshippers of Som Addream, the God of Healing) or the ability to create biscuits out of nowhere (worshippers of Geoff, the God of Biscuits).
The symbol of the God of Gods is five circles, placed haphazardly on top of each other.
Due to the nature of All-That-Is religion, all gods claim to be the One True God, and thus no official church statements have been made by the Voices specifically regarding this particularly odd religion, however the people of All-That-Is regard it as a rather odd, but harmless religion.
Fantasy city by David Revoy, licensed under the
Creative Commons Attribution 3.0 Unported license.