Protestant vs Christian

Protestant and Christian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Protestant	Yes	Yes	No	No
Christian	Yes	Yes	No	No

As nouns, Christian is a hypernym of Protestant; that is, Christian is a word with a broader meaning than Protestant:

Protestant: an adherent of Protestantism
Christian: a religious person who believes Jesus is the Christ and who is a member of a Christian denomination

Protestant (noun)	Christian (noun)
the Protestant churches and denominations collectively	a religious person who believes Jesus is the Christ and who is a member of a Christian denomination
an adherent of Protestantism

Protestant (adjective)	Christian (adjective)
of or relating to Protestants or Protestantism	relating to or characteristic of Christianity