Roman Catholic vs R.C.

Roman Catholic is a noun but R.C. is not a noun.

Word	Noun	Adjective	Verb	Adverb
Roman Catholic	Yes	Yes	No	No
R.C.	No	Yes	No	No

As adjectives, Roman Catholic and R.C. are synonyms defined as:

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

Roman Catholic (noun)	R.C. (noun)
the Christian Church based in the Vatican and presided over by a pope and an episcopal hierarchy
a member of the Roman Catholic Church

Roman Catholic (adjective)	R.C. (adjective)
of or relating to or supporting Romanism	of or relating to or supporting Romanism