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.
<u>"A Famous Protestant</u>" 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 <u>lovely and tiny</u> cake", "<u>The orange and modern</u> car is not here anymore" "The exam was <u>difficult and very long</u>", etc.