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; for the Creator of man is also the Founder of human societies, and preserves their existence as He preserves our own. We owe Him, therefore, not only a private cult, but a public and social worship to honor Him....Hence the Roman Pontiffs have never ceased, as circumstances required, to refute and condemn the doctrine of the separation of Church and State. Our illustrious predecessor, Leo XIII, especially, has frequently and magnificently expounded Catholic teaching on the relations which should subsist between the two societies. "Between them," he says, "there must necessarily be a suitable union, which may not improperly be compared with that existing between body and soul." He proceeds: "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." (Pope St. Pius X, Vehementer Nos (On the French Law of Separation))
55. The Church ought to be separated from the State, and the State from the Church. (Bl. Pius IX, Syllabus of Errors)
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. (ibid.)
Anyone who tries to make Christ more than a spiritual King is deluding himself. Jesus Himself said "My kingdom is not of this world."
He undoeth that which Pilate for a while had feared, namely, the suspicion of seizing kingly power, "Is then His kingdom not of this world also?"45 Certainly it is. "How then saith He it `is not'?" Not because He doth not rule here, but because He hath his empire from above, and because it is not human, but far greater than this and more splendid. "If then it be greater, how was He made captive by the other?" By consenting, and giving Himself up. But He doth not at present reveal46 this, but what saith He? "If I had been of this world, `My servants would fight, that I should not be delivered.'" Here He showeth the weakness of kingship among us, that its strength lies in servants; but that which is above is sufficient for itself, needing nothing. From this the heretics taking occasion say, that He is different from the Creator. What then, when it saith, "He came to His own"? (c. i. 11.) What, when Himself saith, "They are not of this world, as I am not of this world"? (c. xvii. 14.) So also He saith that His kingdom is not from hence, not depriving the world of His providence and superintendence, but showing, as I said, that His power was not human or perishable. What then said Pilate? (St. John Chrysostom, Homilies on the Gospel of John LXXXIII)
And from Jesus Christ, who is the faithful witness, the first begotten of the dead and the prince of the kings of the earth, who hath loved us and washed us from our sins in his own blood And hath made us a kingdom, and priests to God and his Father. To him be glory and empire for ever and ever. Amen. (Revelation of St. John 1:5-6)
17. 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 IX, Quas Primas (On the Feast of Christ the King))
Theocracies have traditionally punished anyone who doesn't agree with the religious worldview of the theocrat.
From which totally false idea of social government they do not fear to foster that erroneous opinion, most fatal in its effects on the Catholic Church and the salvation of souls, called by Our Predecessor, Gregory XVI, an "insanity,"2 viz., that "liberty of conscience and worship is each man's personal right, which ought to be legally proclaimed and asserted in every rightly constituted society; and that a right resides in the citizens to an absolute liberty, which should be restrained by no authority whether ecclesiastical or civil, whereby they may be able openly and publicly to manifest and declare any of their ideas whatever, either by word of mouth, by the press, or in any other way." But, while they rashly affirm this, they do not think and consider that they are preaching "liberty of perdition;" and that "if human arguments are always allowed free room for discussion, there will never be wanting men who will dare to resist truth, and to trust in the flowing speech of human wisdom; whereas we know, from the very teaching of our Lord Jesus Christ, how carefully Christian faith and wisdom should avoid this most injurious babbling."...
Therefore, by our Apostolic authority, we reprobate, proscribe, and condemn all the singular and evil opinions and doctrines severally mentioned in this letter, and will and command that they be thoroughly held by all children of the Catholic Church as reprobated, proscribed and condemned. [This fulfills the conditions for an ex cathedra definition] (Bl. Pius IX, Quanta Cura)
That is exactly my take on it. I don't know who originated that idea.
Oh, just Gregory XVI, Bl. Pius IX, Leo XIII, St. Pius X, and Pius XI. Guess we can just ignore all of them, right?