15 The one only reason which men have for not obeying is when anything is demanded of them which is openly repugnant to the natural or the divine law, for it is equally unlawful to command to do anything in which the law of nature or the will of God is violated. If, therefore, it should happen to any one to be compelled to prefer one or the other, viz., to disregard either the commands of God or those of rulers, he must obey Jesus Christ, who commands us to "give to Caesar the things that are Caesar's, and to God the things that are God's,"(18) and must reply courageously after the example of the Apostles: "We ought to obey God rather than men."(19) And yet there is no reason why those who so behave themselves should be accused of refusing obedience; for, if the will of rulers is opposed to the will and the laws of God, they themselves exceed the bounds of their own power and pervert justice; nor can their authority then be valid, which, when there is no justice, is null.
Source: Diuturnum (Vatican.va)