Replies

Separation of church and state is false and has been condemned many times by the Church.

55. The Church ought to be separated from the State, and the State from the Church.—Allocution 'Acerbissimum,' Sept. 27, 1852.
56. Moral laws do not stand in need of the divine sanction, and it is not at all necessary that human laws should be made conformable to the laws of nature and receive their power of binding from God.—Allocution 'Maxima quidem,' June 9, 1862.
57. The science of philosophical things and morals and also civil laws may and ought to keep aloof from divine and ecclesiastical authority.—Ibid.
-- Pope Bl. Pius IX, The Syllabus of Errors

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 reaching 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.
-- Pope Leo XIII, Encyclical "Immortale Dei"

That the State must be separated from the Church is a thesis absolutely false, a most pernicious error. Based, as it is, on the principle that the State must not recognize any religious cult, it is in the first place guilty of a great injustice to God...Hence the Roman Pontiffs have never ceased, as circumstances required, to refute and condemn the doctrine of the separation of Church and State.
-- Pope St. Pius X, Encyclical "Vehementer nos"

It would be a grave error, on the other hand, to say that Christ has no authority whatever in civil affairs, since, by virtue of the absolute empire over all creatures committed to him by the Father, all things are in his power.
-- Pope Pius XI, Encyclical "Quas Primas"

O Christ, that Heads of State may honor Thee publicly, that professors and judges honor Thee, that Thou be manifest in laws and the arts!
-- Vespers Hymn of Christ the King

Therefore it leaves untouched traditional Catholic doctrine on the moral duty of ... societies toward the true religion and toward the one Church of Christ.
-- Vatican II, Declaration "Dignitatis Humanae"