founder of a religious sect
Refers specifically to the person who established a religious group or denomination. Can also be used metaphorically for the originator of a movement or school of thought.
The founder of this sect was a monk in the Edo period.
He is regarded as the founder of a new spiritual movement.
He became the founder of a new religious sect.
The man called himself a cult leader and gathered followers.
The followers revere him as their cult leader.
宗祖 is a formal term used mainly for the founders of Buddhist sects, while 教祖 is a broader, more everyday word for the founder of any religious group, often carrying a slightly negative 'cult leader' connotation.
教主 refers to the current head or leader of a religion, whereas 教祖 specifically means the person who originally founded it.
教祖 is the leader who founds a religious group, while 信者 is a follower or believer who follows that leader.
教祖 is a person, so it is counted with 人, as in 一人の教祖.
教 (teaching, religion) + 祖 (ancestor, founder). The compound straightforwardly means 'religious founder'.