To Play Devil’s Advocate is to be in the position of presenting a hypothetical argument to a suggestion or proposal. The source of this expression lies with the Catholic Church and the process of proposing a name for canonization, which is to make an individual into a saint.
When a name is proposed, a member of the clergy is appointed to present the opposing argument and that person is known as the advocatus diaboli, from the Latin words advocatus, meaning ‘summoned one’, and diaboli, ‘of the devil’. Thus the ‘devil’s advocate’ has been summoned from the devil to present his argument. His opponent in the discusisons is known as the Advocatus Dei (‘God’s advocate’), who presents the case in favour of beatification or canonization.