An adjective phrase is a group of words that describe a noun or pronoun, they can be located before or after the noun or pronoun in the sentence.
"A Famous Protestant" is the correct sentence because it describes the "preacher", in this case the adjective phrase is located before the noun.
Some examples of an adjective phrase are "That’s a lovely and tiny cake", "The orange and modern car is not here anymore" "The exam was difficult and very long", etc.
