devil's advocate

One who argues against a cause or position either for the sake of argument or to help determine its validity. For example, My role in the campaign is to play devil's advocate to each new policy before it's introduced to the public. This term comes from the Roman Catholic Church, where advocatus diaboli (Latin for “devil's advocate”) signifies an official who is appointed to present arguments against a proposed canonization or beatification. It was transferred to wider use in the mid-1700s.