What term is used for a small, secretive religious group representing a new religion or a major change in an existing faith?

The term that describes a small, secretive religious group that signifies either a new religion or a significant transformation within an existing faith is known as a new religious movement (NRM), often referred to colloquially as a cult. NRMs typically emerge in response to perceived deficiencies or failures in established religious traditions, and they often attract followers who are seeking new spiritual paths or answers to life's questions. These groups are characterized by their innovation in beliefs and practices, distinct organizational structures, and sometimes their exclusiveness or secrecy regarding their teachings.

In contrast, the term "traditional church" refers to established, mainstream religious institutions that have a long history and generally adhere to established doctrines. An "established denomination" also represents a recognized branch within a larger religious tradition, with established practices and social acceptance that differ from the novelties of NRMs. A "revolutionary sect" might imply a group that seeks to radically reform or challenge established religious orders but does not capture the broader definition that NRMs encompass regarding entirely new belief systems or considerable shifts in existing ones. Thus, the label of new religious movement (NRM) or cult is the most fitting for small, secretive religious groups marked by new or drastically altered religious ideas.