be (the) devil's advocate

To argue against or attack 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 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.