My own studies of Buddhism includes a few years of regular training in the art of zen sword at a Buddhist temple in my area. The master was a Korean who displayed astonishing sword skills. His bio says that he attained enlightenment with his vision of this martial art, called Shim Gum Go ("Mind Sword Path) during a 90-day solo meditation on a mountain top in South Korea. Having lost his rice provisions on his way up the mountain to begin his meditation, he battled virtual starvation throughout the next three months.
Anyway, the official Proclamation of Shim Gum Do includes the following language:
Avatamsaka Sutra says, "If you want to understand that all the three worlds are Buddha, you must perceive world substance. All things are created by mind alone." This means that if you want to understand the true way you must perceive where name and form come from and you must understand that name and form are created by mind. In this world, one by one, each thing is complete; one by one, each thing has substance. If you cut off all thinking, return to before thinking then this is your substance and universal substance. We call this 'primary point'. If you keep this mind, you and everything, you and the universe, become one. Clear like space, without name and form, without opposites, that is the Absolute. We call it Mind or Buddha or God or Truth or Energy. This is Shim.
I would submit that the concept of Mind as the creative fore of everything qualifies as a belief in what some have called "the Universal Mind Substance," which is close enough to a Supreme Being for purposes of my definition.
The nature of our relationship with that Mind may be completely different from the Judeo-Christian concepts relating to God.
But based on my study and admittedly limited understanding, I believe that Buddhism nevertheless does have at its core some recognition of a supreme creative force, and thus qualifies as a religion.
However, using that argument, to some degree, communism, etc., also believe in the society as the supreme being. (Durkheim, IIRC, argues that religion is actually the manifest of the society itself). So, it brings back to my first posting that questioning why 'religion', seen as a philosophy, is single out as 'bad'.