Why is someone who challenges what appears to be an obvious truth called a “devil’s advocate”?

During the Roman Catholic proceedings leading to the assignment of
sainthood, a specific individual is given the job of investigating the can-
didate and the validity of any associated miracles. He then argues vehe-
mently against the canonization by denigrating the potential saint on
behalf of the devil. His official Vatican title is the “Devil’s Advocate.”