The belief that all each religion has some truth to it.
It is difficult to ascertain if it is a form of religion or irreligion. However, since it lacks inherent ceremonies, I would say it is a form of irreligion.
Omnists generally respect most faiths as they seem all as true.
Omnists are separate from atheits as Omnists believe in everything, while atheists believe in nothing.
Chris Martin of Coldplay is one, although he describes himself as an All-theist. He probably hasn't heard of Omnism