Here's why:
* Common noun: A common noun refers to a general class of people, places, or things (e.g., dog, city, book). "Religion" refers to a general concept, not a specific instance.
* Collective noun: A collective noun names a group of people, animals, or things (e.g., team, flock, committee). While religions can be grouped together, "religion" itself isn't a group, but a concept.
Examples:
* Common noun: "The study of religion is fascinating." (refers to the general concept)
* Collective noun: "The council of religions met to discuss peace." (refers to a group of religious institutions)