Religion is a very diverse group of beliefs and practices that span all cultures around the world. Some believe that religion names a universal feature of human existence, while others have doubts. A large number of scholars, influenced by Foucauldian or post-colonial theory, have criticised the concept for its involvement in Western statism and imperialism. This criticism is not of the faiths and worldviews of self-described religious individuals or groups, but rather of the concept of religion itself.
These scholars argue that one must abandon the classical notion of a social taxon (which is based on the assumption that every instance accurately described by a concept will have some defining property) and instead use the prototypical idea of a category, which allows different instances to have very similar characteristics. This approach is more suited to the study of complex phenomena.
It also provides a way to make sense of the variety of religions, even if they do not share a common defining property. This is important in comparative religion, as well as in the study of religious diversity more generally.
One way to understand the many different religions is to read their holy books. This is a very useful way to see the many different approaches that they take to various concepts, including truth, scripture, behavior, and reason. Another great way to learn more about a religion is to talk with someone of that religion. If you are interested in studying a particular religion, please speak to your advisor about taking courses with a professor who studies that religion.