sacred text vs written material

sacred text and written material both are nouns.

Word	Noun	Adjective	Verb	Adverb
sacred text	Yes	No	No	No
written material	Yes	No	No	No

As nouns, written material is a hypernym of sacred text; that is, written material is a word with a broader meaning than sacred text:

sacred text: writing that is venerated for the worship of a deity
written material: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of sacred text include piece of writing, writing.

sacred text (noun)	written material (noun)
writing that is venerated for the worship of a deity	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)