Let’s tone down the rhetoric, and not take things so personally. An interesting discussion on the important topic of what a religion is doesnt need to devolve in charges of religious extremism, which in a group like this, is a big insult indeed
Now as for what a religion is:
Whatever else it is, it has a supernatural component.
At the turn of the century, the high court ruled that religion is “faith in god.” But that definition was soon found to be too narrow, since there are religions that dont have concepts of faith and others that dont have concepts of a god (pantheisms and panentheisms, etc.)
Soon after, the consensus definition of religion was a functionalist one: if it looks like a religion, and quacks like a religion, it is a religion. But this definition was found to be too broad—functionalists defined Marxism as a religion, going to baseball games as religious (everyone adheres to certain ritualized elements, the convocation begins with a hymn, there are heights of emotion like in some religious services, etc.)
Another far too broad definition is Paul Tillich’s: that religion and God is the “object of one’s ultimate concern.” But this is also far too broad: it allows that money can be some people’s religion. Or sex. Or any obsession.
A definition that includes everything fails to draw distinctions and loses its effectiveness as a definition.
So the consensus definition of religion now the last 30 or so years among scholars in the field who study it is: whatever elese it is, it has a supernatural component. Thus, Mahayana and Theraveda Buddhism are considered religions (belief in supernatural claims regarding the life of the Buddha, ancestor worship, etc.) but Zen is not, as it is without supernatural belief. That’s why scholars call Zen and certain kinds of confucianism also “Eastern Philosophy”
There are other things they say helps define religion: that it is participatory, at least by the individual, that it is a “communication system,” and that there is an other-worldly component. Etc.