religious song vs hymn

religious song and hymn both are nouns.

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

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

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

religious song (noun)	hymn (noun)
religious music for singing	a song of praise (to God or to a saint or to a nation)

religious song (verb)	hymn (verb)
	praise by singing a hymn
	sing a hymn