Roman vs Roman Catholic

Roman and Roman Catholic both are nouns.

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

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

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

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

Roman (adjective)	Roman Catholic (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