a/the devil's advocate
One who argues against or attacks an idea, argument, or proposition—even if one is in favor of it—for the sake of debate or to further examine its strength, validity, or details. Refers to the "Advocatus Diaboli," a person employed by the Catholic Church to argue against the canonization of a saint (and therefore help determine if that person is truly worthy of sainthood). I'm all for universal health care, but I'll be the devil's advocate in asking how the government intends to fund such a massive undertaking. Tom is always the devil's advocate in any given conversation because he loves picking apart other people's arguments.
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.