"The doctrine on the necessary conformity of civil law with the moral law is in continuity with the whole tradition of the Church. This is clear once more from John XXIII's Encyclical: 'Authority is a postulate of the moral order and derives from God. Consequently, laws and decrees enacted in contravention of the moral order, and hence of the divine will, can have no binding force in conscience...; indeed, the passing of such laws undermines the very nature of authority and results in shameful abuse'.95 This is the clear teaching of Saint Thomas Aquinas, who writes that 'human law is law inasmuch as it is in conformity with right reason and thus derives from the eternal law. But when a law is contrary to reason, it is called an unjust law; but in this case it ceases to be a law and becomes instead an act of violence'.96 And again: 'Every law made by man can be called a law insofar as it derives from the natural law. But if it is somehow opposed to the natural law, then it is not really a law but rather a corruption of the law'.97"
Pope John Paul II, Evangelium vitae (25 March 1995) 72.