Roman Catholic vs Roman

Roman Catholic and Roman both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roman Catholic	Yes	Yes	No	No
Roman	Yes	Yes	No	No

As adjectives, Roman Catholic and Roman are synonyms defined as:

Other synonyms of Roman Catholic include papist, papistic, papistical, popish, R.C., Romanist, romish.

Roman Catholic (noun)	Roman (noun)
the Christian Church based in the Vatican and presided over by a pope and an episcopal hierarchy	a resident of modern Rome
a member of the Roman Catholic Church	an inhabitant of the ancient Roman Empire

Roman Catholic (adjective)	Roman (adjective)
of or relating to or supporting Romanism	of or relating to or supporting Romanism
	of or relating to or derived from Rome (especially ancient Rome)
	characteristic of the modern type that most directly represents the type used in ancient Roman inscriptions