In America, the idea of separation of Church and State is widely accepted; even among Catholics this concept is largely endorsed, in spite of the fact that it has been condemned again and again by the Church.
According to a 2017 Pew Research survey:
In this week's Mic'd Up, Michael Voris interviews David Wemhoff, author of John Courtney Murray, Time/Life, and the American Proposition.
The Josias is a manual of Catholic Integralism that attempts to articulate a truly Catholic political stance from which to approach the present order of society. It defines Catholic Integralism as:
A tradition of thought that, rejecting the liberal separation of politics from concern with the end of human life, holds that political rule must order man to his final goal. Since, however, man has both a temporal and an eternal end, integralism holds that there are two powers that rule him: a temporal power and a spiritual power. And since man's temporal end is subordinated to his eternal end, the temporal power must be subordinated to the spiritual power.
This idea is also explained in great detail in the book Integralism: A Manual of Political Philosophy.
This is the tradition of the Church infallibly taught by numerous popes.
Pope Gelasius wrote a letter to Emperor Anastasius in the year 494, saying:
There are two powers, august Emperor, by which this world is chiefly ruled, namely, the sacred authority of the priests and the royal power. Of these, that of the priests is the more weighty since they have to render an account for even the kings of men in the divine judgment.
Likewise, Pope Pius X, in his 1906 encyclical Vehementer Nos, stated, "That the State must be separated from the Church is a thesis absolutely false, a most pernicious error."
On the contrary, 20th century Jesuit theologian John Courtney Murray was one of the biggest proponents of the separation of Church and State under the guise of "religious liberty." He, along with other liberal Churchmen, promoted religious indifferentism, which equates all religions (not recognizing that Catholicism is the One True Faith).
Catholicism in practice is both a private and public matter. Just as one cannot truly practice his faith if it's restricted merely to one's private life, so too the Catholic Church cannot have absolute moral authority in just one arena.
One must practice his faith publicly just as the Church must determine good and evil in every aspect of life — especially politically, for that's where laws are made that then affect the whole community.
If the Church is the only authority that can determine moral right and wrong, which it is, it follows that because politics involves moral decisions, the Church cannot be separated from these decisions.
Watch the full episode of Mic'd Up — Separation of Church and State.