uses as both noun and adjective

I once posted a message inquiring about some element of grammar that appears frequently which acts as an adjective when it has the appearance of a noun and a reply came in stating that you cannot use every noun as an adjective but some nouns can be used as adjectives because they can assume/carry both functions, as a noun and an adjective. OK but my intuition tells me that might not be all there is. Help.

I am Christian. (OK, no article here because it can function in both ways.)

I am Chinese. (OK, similar to the above case)

It was both home and fortress. (What are the words "home" and "fortress"? adjectives? nouns?)