No. You do not need to be religious to take IB World Religions SL. The course is an academic study of religion, so assessment focuses on knowledge, analysis, empathy and evidence rather than personal belief or participation in worship.
You study how religious beliefs and practices shape the lives of adherents. This includes the three fundamental questions: what is the human condition, where are we going, and how do we get there? You learn how different traditions answer these questions, while recognizing internal diversity between denominations, schools, cultures and regions.
Being non-religious does not prevent you from understanding a believer's perspective. The course expects an insider and outsider approach: you should represent what adherents believe accurately and empathetically, then analyse those beliefs as a student of religion. For example, you might explain how Theravada Buddhist ideas about anatta relate to practice, or how Orthodox and Reform Jewish interpretations of mitzvot differ, without claiming that either interpretation is universally held.
You also do not need prior religious education. In Part 1, schools study five of nine religions through Paper 1. In Part 2, students study two religions in depth for Paper 2, and the internal assessment is an investigative study of religious experience, practice or belief. The investigation can focus on any religion, provided it is researched ethically and presented objectively.
For exam technique, avoid writing personal opinions such as “I believe this is true” or “that religion is irrational.” Attribute claims precisely: “Some Sunni Muslims believe...” or “Roman Catholic teaching holds...” Then connect beliefs to behaviour, rituals, texts and contemporary contexts. Examiners reward accurate terminology, specific examples and balanced analysis, not personal faith.