religious cult vs cultus

religious cult and cultus both are nouns.

Word	Noun	Adjective	Verb	Adverb
religious cult	Yes	No	No	No
cultus	Yes	No	No	No

As nouns, religious cult and cultus are synonyms defined as:

Other synonyms of religious cult include cult.

religious cult (noun)	cultus (noun)
a system of religious beliefs and rituals	a system of religious beliefs and rituals