The advocatus diaboli, as you know, is not a lawyer employed by the Prince of Darkness. He is a faithful member of the Church whose duty it is, when it is proposed to canonize a saint, to search out all the opposing considerations and to state them as cogently as possible. This wise institution compels the advocates of canonization to exert themselves to develop arguments vigorous enough to overcome all objections. In this symposium on religion, I am asked to serve as advocatus diaboli: to state the Dark Side so that those who follow may have definite positions to attack and may thus more fully develop the strength of their case.
Source: The Faith of a Liberal (NY: Henry Holt, 1946)
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