30 Another liberty is widely advocated, namely, liberty of conscience . If by this is meant that everyone may, as he chooses, worship God or not, it is sufficiently refuted by the arguments already adduced. But it may also be taken to mean that every man in the State may follow the will of God and, from a consciousness of duty and free from every obstacle, obey His commands. This, indeed, is true liberty, a liberty worthy of the sons of God, which nobly maintains the dignity of man and is stronger than all violence or wrong - a liberty which the Church has always desired and held most dear. This is the kind of liberty the Apostles claimed for themselves with intrepid constancy, which the apologists of Christianity confirmed by their writings, and which the martyrs in vast numbers consecrated by their blood. And deservedly so; for this Christian liberty bears witness to the absolute and most just dominion of God over man, and to the chief and supreme duty of man toward God. It has nothing in common with a seditious and rebellious mind; and in no title derogates from obedience to public authority; for the right to command and to require obedience exists only so far as it is in accordance with the authority of God, and is within the measure that He has laid down. But when anything is commanded which is plainly at variance with the will of God, there is a wide departure from this divinely constituted order, and at the same time a direct conflict with divine authority; therefore, it is right not to obey.
Source: Libertas (Vatican.va)