30 The Church alike and the State, doubtless, both possess individual sovereignty; hence, in the carrying out of public affairs, neither obeys the other within the limits to which each is restricted by its constitution. It does not hence follow, however, that Church and State are in any manner severed, and still less antagonistic, Nature, in fact, has given us not only physical existence, but moral life likewise. Hence, from the tranquillity of public order, which is the immediate purpose of civil society, man expects to derive his well-being, and still more the sheltering care necessary to his moral life, which consists exclusively in the knowledge and practice of virtue. He wishes, moreover, at the same time, as in duty bound, to find in the Church the aids necessary to his religious perfection, in the knowledge and practice of the true religion; of that religion which is the queen of virtues, because in binding these to God it completes them all and perfects them. Therefore, they who are engaged in framing constitutions and in enacting laws should bear in mind the moral and religious nature of man, and take care to help him, but in a right and orderly way, to gain perfection, neither enjoining nor forbidding anything save what is reasonably consistent with civil as well as with religious requirements. On this very account, the Church cannot stand by, indifferent as to the import and significance of laws enacted by the State; not insofar, indeed, as they refer to the State, but in so far as, passing beyond their due limits, they trench upon the rights of the Church.
Source: Sapientiae Christianae (Vatican.va)