Roman Catholic vs Western Church

Roman Catholic and Western Church both are nouns.

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

As nouns, Roman Catholic and Western Church are synonyms defined as:

Roman Catholic and Western Church: the Christian Church based in the Vatican and presided over by a pope and an episcopal hierarchy

Other synonyms of Roman Catholic include Church of Rome, Roman Catholic Church, Roman Church.

Roman Catholic (noun)	Western Church (noun)
the Christian Church based in the Vatican and presided over by a pope and an episcopal hierarchy	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)	Western Church (adjective)
of or relating to or supporting Romanism