religious song vs carol

religious song and carol both are nouns.

Word	Noun	Adjective	Verb	Adverb
religious song	Yes	No	No	No
carol	Yes	No	Yes	No

As nouns, carol is a hyponym of religious song; that is, carol is a word with a more specific, narrower meaning than religious song:

Other hyponyms of religious song include chant, Negro spiritual, spiritual, Christmas carol, anthem, hymn.

religious song (noun)	carol (noun)
religious music for singing	a joyful song (usually celebrating the birth of Christ)
	joyful religious song celebrating the birth of Christ

religious song (verb)	carol (verb)
	sing carols