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Such factors as religious service attendance, belief, practice, familiarity with doctrine, belief in certain creeds, etc., may be important to sociologists, religious leaders, and others. But these are measures of religiosity and are usually not used academically to define a person’s membership in a particular religion. It is important to recognize there are various levels of adherence, or membership within religious traditions or religious bodies. There’s no single definition, and sources of adherent statistics do not always make it clear what definition they are using.