Acting as Devil's advocate

Arguing against a proposed decision of an individual or group by using their own methods, beliefs, and laws; the intent being to have them look at that decision from another perspective.

In the beatification of saints in the Roman Catholic Church, the Devil's Advocate is the official of the Vatican Congregation of Rites whose responsibility is to guard the welfare, beliefs, and laws of the Church. He is in direct opposition to the "Saint's Advocate", assigned to guard the progress of the applicant for canonization.