religious writing vs Upanishad

religious writing and Upanishad both are nouns.

Word	Noun	Adjective	Verb	Adverb
religious writing	Yes	No	No	No
Upanishad	Yes	No	No	No

religious writing (noun)	Upanishad (noun)
writing that is venerated for the worship of a deity	a later sacred text of Hinduism of a mystical nature dealing with metaphysical questions