An Unlawful Oath Is Not Binding

"An oath cannot bind to that which is in itself unlawful, because the obligation of the law is imposed upon us by the will of God, and therefore takes precedence of all obligations imposed upon us by the will of men or by ourselves; and the lesser obligation cannot relieve from the greater. The sin is in taking the oath to do the unlawful thing, not in breaking it. Therefore Luther was right in breaking his monastic vows."

--A. A. Hodge [1869], from The Westminster Confession: A Commentary (Banner of Truth, 2002, p. 289)