| Preach to the choir|
|Crux of the matter|
|Speak of the devil|
|An act of faith|
Omnitheism (also called Omnism) is belief in the existence of all possible gods. Atheists believe in no gods; monotheists believe in one god; polytheists believe in many but not necessarily all gods; omnitheists believe in the existence of all gods that can possibly be believed in.
Consider this list of gods whom Christians and atheists do not believe in. Omnitheists believe in the existence of every god on that list; yes, that includes Suzumiya Haruhi. There are also many other deities in which omnitheists believe and which that list does not include such as (presumably) the Flying Spaghetti Monster and the Invisible Pink Unicorn.
By extension they (again presumably) also believe in "dead gods" - gods who now nobody else believes in - such as the ancient Egyptian pantheon.
The God Om
"Om" is also one of the the gods of the discworld where he is heavily into the Judeo-Christian tradition of smiting, manifestations, monotheism, churches, sermons, inquisitions and commandments. His followers are called "Omnians". Presumably Omnitheists, and Omnists, would also be Omnians as they would also believe in Om.