cultus vs religious cult

cultus and religious cult both are nouns.

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

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

Other synonyms of cultus include cult.

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