A system of belief is a set of principles or ideas that guide an individual's behavior and decisions. Creed, CREDO, ISM, and RELIGION are all terms used to describe a system of belief. A creed is a statement of faith or belief, while a CREDO is a statement of principles or beliefs. An ISM is a suffix used to denote a particular system of belief, and RELIGION is a set of beliefs concerning the cause, nature, and purpose of the universe.