"Cult" is to faith/religion/church as "neurosis" is to psychology. It has a such a broad meaning that virtually anyone in psychology, and any organization in faith/religion/church, qualifies.
Nevertheless, my own meaning would be this in regard to the Christian faith:
An organization, or at least a unified purpose, of religious doctrine and practice characterized by an attack on longer established assemblies and doctrines of Christian faith, by claiming to be the only channel by which the Supreme Being communicates with people today-- either by direct revelation or by possessing the only flawless understanding of the scriptures-- by having a strong emphasis on wedge issues; that is, beliefs and practices that the established assemblies have thought or practiced in the "wrong" way [the sabbath, dietary restrictions, blood transfusions, what happens to the soul after physical death,....], and finally by requiring commitments by members to regard the organization or unified purpose as the top priority in a person's life, including requiring to various degrees that a member cut ties with acquaintances or family members who may influence the member in another direction.
How's that for a sentence?