“Men living together in society are under the power of God no less than individuals are, and society, not less than individuals, owes gratitude to God, who gave it being and maintains it, and whose ever-bounteous goodness enriches it with countless blessings. Since, then, no one is allowed to be remiss in the service due to God, and since the chief duty of all men is to cling to religion in both its teaching and practice—not such religion as they may have a preference for, but the religion which God enjoins, and which certain and most clear marks show to be the only one true religion—it is a public crime to act as though there were no God. So, too, is it a sin in the State not to have care for religion, as a something beyond its scope, or as of no practical benefit; or out of many forms of religion to adopt that one which chimes in with the fancy; for States are bound absolutely to worship God in that way which He has shown to be His will. All who rule, therefore, should hold in honour the holy Name of God, and one of their chief duties must be to favour religion, to protect it . . . . “
~ Pope Leo XIII
The doctrine of the separation of church and state, which is extolled in America by both the religious and non-religious alike, for opposite reasons, is the political result of the victory of the temporal power over the spiritual authority. For there is no such thing as “separate but equal,” and once separation by law becomes institutional, then the party responsible for making and maintaining the law immediately rises to supremacy.
In the same Encyclical he cites as reprehensible these views:
The State (civitas) does not consider itself bound by any kind of duty towards God. Moreover, it believes that it is not obliged to make public profession of any religion; or to inquire which of the very many religions is the only one true; or to prefer one religion to all the rest; or to show to any form of religion special favour; but, on the contrary, is bound to grant equal rights to every creed, so that public order may not be disturbed by any particular form of religious belief.
Again, in his Encyclical Libertas, 20 June 1888, he teaches:
This kind of liberty [liberty of cult], if considered in relation to the State, clearly implies that there is no reason why the State should offer any homage to God, or should desire any public recognition of Him; that no one form of worship is to be preferred to another, but that all stand on an equal footing, no account being taken of the religion of the people, even if they profess the Catholic faith…. Civil society [civilis societas, quia societas est] must acknowledge God as its Founder and Parent, and must obey and reverence His power and authority. Justice therefore forbids, and reason itself forbids, the State to be godless; or to adopt a line of action which would end in godlessness—namely, to treat the various religions (as they call them) alike, and to bestow upon them promiscuously equal rights and privileges. Since, then, the profession of one religion is necessary in the State, that religion must be professed which alone is true, and which can be recognized without difficulty, especially in Catholic States, because the marks of truth are, as it were, engraven upon it.
Likewise, Pius X wrote in his Encyclical Vehementer nos, ll February 1906:
That the State should be separated from the Church is an absolutely false and most pernicious thesis. For first, since it is based on the principle that religion should be of no concern to the State, it does a grave injury to God, He Who is the founder and conserver of human society no less than He is of individual men, for which reason He should be worshipped not only privately but also publicly.
 Immortale Dei.