Great question -The definition of philosophy is :the study of the fundamental nature of knowledge, reality, and existence, esp. when considered as an academic discipline. I would say religion is a sub-category of philosophy because philosophy studies existence. How many times have you seen the question "Does God exist?" I took Philosophy of Religion also in college and it was interesting to learn about other religions, their practices, what god they believe in, and how some of our practices, beliefts, etc are similar. So I think when talking about philosophy, religion is one aspect of it. But, in general conversation, the two are very different categories.