This term is generally used to describe someone who takes the unpopular (opposite) side in an argument (either out of contentiousness or out of a zeal for the truth).

This epithet originally applied to the Vatican official charged with finding objections to beatification or canonization; or, in other words, it was his responsibility to argue against the sainthood of a candidate who was being considered.

Since the 16th century, this individual's title has been promotor fidei, "promotor of the faith" as expressed in the Catholic Encyclopedia which clarifies his function as follows: "The seemingly negative work of the promoter of the faith undoubtedly has a great positive value, in as much as it prevents the Church from pronouncing a certain and favorable judgment on the life and works of a person without possessing unquestionable proof."