lettered vs knowing

lettered is not a noun while knowing is a noun.

Word	Noun	Adjective	Verb	Adverb
lettered	No	Yes	No	No
knowing	Yes	Yes	No	No

As adjectives, lettered and knowing are synonyms defined as:

lettered and knowing: highly educated; having extensive information or understanding

Other synonyms of lettered include knowledgeable, learned, well-educated, well-read.

lettered (noun)	knowing (noun)
	a clear and certain mental apprehension

lettered (adjective)	knowing (adjective)
highly educated; having extensive information or understanding	highly educated; having extensive information or understanding
	evidencing the possession of inside information
	characterized by conscious design or purpose
	alert and fully informed