Libertas §18

¹⁸ There are others, somewhat more moderate though not more consistent, who affirm that the morality of individuals is to be guided by the divine law, but not the morality of the State, for that in public affairs the commands of God may be passed over, and may be entirely disregarded in the framing of laws. Hence follows the fatal theory of the need of separation between Church and State. But the absurdity of such a position is manifest. Nature herself proclaims the necessity of the State providing means and opportunities whereby the community may be enabled to live properly, that is to say, according to the laws of God. For, since God is the source of all goodness and justice, it is absolutely ridiculous that the State should pay no attention to these laws or render them abortive by contrary enact menu. Besides, those who are in authority owe it to the commonwealth not only to provide for its external well-being and the conveniences of life, but still more to consult the welfare of men's souls in the wisdom of their legislation. But, for the increase of such benefits, nothing more suitable can be conceived than the laws which have God for their author; and, therefore, they who in their government of the State take no account of these laws abuse political power by causing it to deviate from its proper end and from what nature itself prescribes. And, what is still more important, and what We have more than once pointed out, although the civil authority has not the same proximate end as the spiritual, nor proceeds on the same lines, nevertheless in the exercise of their separate powers they must occasionally meet. For their subjects are the same, and not infrequently they deal with the same objects, though in different ways. Whenever this occurs, since a state of conflict is absurd and manifestly repugnant to the most wise ordinance of God, there must necessarily exist some order or mode of procedure to remove the occasions of difference and contention, and to secure harmony in all things. This harmony has been not inaptly compared to that which exists between the body and the soul for the well-being of both one and the other, the separation of which brings irremediable harm to the body, since it extinguishes its very life.