Sed contra: (1) There is what the ever-more unpopular, yet irreformable teaching of the ordinary magisterium has firmly established, namely, that the separation of Church and State is an error, which doctrine is clearly summarized in Pope Pius IX's Syllabus of Errors (note: the following are condemned propositions):
55. The Church ought to be separated from the State, and the State from the Church. -- Allocution "Acerbissimum," Sept. 27, 1852.
77. In the present day it is no longer expedient that the Catholic religion should be held as the only religion of the State, to the exclusion of all other forms of worship. -- Allocution "Nemo vestrum," July 26, 1855.
78. Hence it has been wisely decided by law, in some Catholic countries, that persons coming to reside therein shall enjoy the public exercise of their own peculiar worship. -- Allocution "Acerbissimum," Sept. 27, 1852.
79. Moreover, it is false that the civil liberty of every form of worship, and the full power, given to all, of overtly and publicly manifesting any opinions whatsoever and thoughts, conduce more easily to corrupt the morals and minds of the people, and to propagate the pest of indifferentism. -- Allocution "Nunquam fore," Dec. 15, 1856.
Further: (2) There is what Pope Gregory XVI says in his encyclical Mirari vos (On Liberalism and Religious Indifferentism):
Nor can We predict happier times for religion and government from the plans of those who desire vehemently to separate the Church from the state, and to break the mutual concord between temporal authority and the priesthood. It is certain that that concord which always was favorable and beneficial for the sacred and the civil order is feared by the shameless lovers of liberty.
Further: (3) Pope Leo XIII says in his encyclical Libertas praestantissimum (On the Nature of Human Liberty):
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.
Further: (4) There is what Pope St. Pius X has to say in Vehementer nos (On the French Law of Separation of Church and State):
That the State must be separated from the Church is a thesis absolutely false, a most pernicious error.… Hence the Roman Pontiffs have never ceased, as circumstances required, to refute and condemn the doctrine of the separation of Church and State.... Human societies cannot, without becoming criminal, act as if God did not exist or refuse to concern themselves with religion, as though it were something foreign to them, or of no purpose to them.... As for the Church, which has God Himself for its author, to exclude her from the active life of the nation, from the laws, the education of the young, the family, is to commit a great and pernicious error.*
*NB: For the full passage, see the post of 11/15/09, "How We Should Interpret Dignitatis humanae."
Further: (5) There is what Pope St. Pius X said, in his letter to the French State, Une fois encore, after the French State passed a law that declared that the property of the Church in France was now belonged to the State:
16. It is easy to see, Venerable Brethren and beloved sons, from what We have just recalled to you, that this law is an aggravation of the Law of Separation [of Church and State], and we can not therefore do otherwise than condemn it.
The Church does not desire, neither ought she to desire, to mix up without a just cause in the direction of purely civil affairs. On the other hand, she cannot permit or tolerate that the state use the pretext of certain laws of unjust regulations to do injury to the rights of an order superior to that of the state, to interfere with the constitution given the Church by Christ, or to violate the rights of God Himself over civil society.
Many believe in or claim that they believe in and hold fast to Catholic doctrine on such questions as social authority, the right of owning private property, on the relations between capital and labor, on the rights of the laboring man, on the relations between Church and State, religion and country, on the relations between the different social classes, on international relations, on the rights of the Holy See and the prerogatives of the Roman Pontiff and the Episcopate, on the social rights of Jesus Christ, Who is the Creator, Redeemer, and Lord not only of individuals but of nations. In spite of these protestations, they speak, write, and, what is more, act as if it were not necessary any longer to follow, or that they did not remain still in full force, the teachings and solemn pronouncements which may be found in so many documents of the Holy See, and particularly in those written by Leo XIII, Pius X, and Benedict XV.
There is a species of moral, legal, and social modernism which We condemn, no less decidedly than We condemn theological modernism.
It is necessary ever to keep in mind these teachings and pronouncements which We have made; it is no less necessary to reawaken that spirit of faith, of supernatural love, and of Christian discipline which alone can bring to these principles correct understanding, and can lead to their observance. This is particularly important in the case of youth, and especially those who aspire to the priesthood, so that in the almost universal confusion in which we live they at least, as the Apostle writes, will not be "tossed to and fro, and carried about with every wind of doctrine by the wickedness of men, by cunning craftiness, by which they lie in wait to deceive." (Ephesians iv, 14)
[This Vatican Council] leaves untouched traditional Catholic doctrine on the moral duty of men and societies toward the true religion and toward the one Church of Christ. Over and above all this, the council intends to develop [read here, 'to develop organically'] the doctrine of recent popes on the inviolable rights of the human person and the constitutional order of society.