Movement leaders such as Rob Bell appropriate set theory as a means of understanding a basic change in the way the Christian church thinks about itself as a group. "Set theory" is a concept in mathematics that allows an understanding of what numbers belong to a group, or set. A bounded set would describe a group with clear "in" and "out" definitions of membership. The Christian church has largely organized itself as a bounded set, those who share the same beliefs and values are in the set and those who disagree are outside.