The elevation of Christianity to a state religion following the conversion of the Emperor Constantine, resulted in the imposition of faith by secular government and resulted in the abuses perpetrated by the Roman Catholic Church, eventually opposed through the processes of the Reformation and the rise of Protestant and secular forces and resulting in the separation of church and state. In many countries legislation continues to favour the historically dominant religion. In Belgium, for example, non-Catholic churches have been restricted architecturally in their visibility from the street.
Christian and democratic institutions are compatible and probably even necessary to one another's existence. Liberty is essential to happiness and prosperity in this world. Constitutional government is essential to that liberty. The preservation of both is contingent on Christian morality informing both voters and leaders. That morality cannot be maintained without firm faith in Christ.
It is an error to consider the Roman Catholic Church as simply another human institution rather than the Body of Christ.
But there is yet another error no less pernicious to the well-being of the nations and to the prosperity of that great human society which gathers together and embraces within its confines all races. It is the error contained in those ideas which do not hesitate to divorce civil authority from every kind of dependence upon the Supreme Being – First Source and absolute Master of man and of society – and from every restraint of a Higher Law derived from God as from its First Source. Thus they accord the civil authority an unrestricted field of action that is at the mercy of the changeful tide of human will, or of the dictates of casual historical claims, and of the interests of a few. Once the authority of God and the sway of His law are denied in this way, the civil authority as an inevitable result tends to attribute to itself that absolute autonomy which belongs exclusively to the Supreme Maker. It puts itself in the place of the Almighty and elevates the State or group into the last end of life, the supreme criterion of the moral and juridical order, and therefore forbids every appeal to the principles of natural reason and of the Christian conscience. We do not, of course, fail to recognize that, fortunately, false principles do not always exercise their full influence, especially when age-old Christian traditions, on which the peoples have been nurtured, remain still deeply, even if unconsciously, rooted in their hearts.
The idea which credits the State with unlimited authority is not simply an error harmful to the internal life of nations, to their prosperity, and to the larger and well-ordered increase in their well-being, but likewise it injures the relations between peoples, for it breaks the unity of supra-national society, robs the law of nations of its foundation and vigor, leads to violation of others' rights and impedes agreement and peaceful intercourse. Now no one can fail to see how the claim to absolute autonomy for the State stands in open opposition to this natural way that is inherent in man (nay, denies it utterly) and therefore leaves the stability of international relations at the mercy of the will of rulers, while it destroys the possibility of true union and fruitful collaboration directed to the general good. (Papal Encyclical, Summi Pontificatus, 20 October 1939).
An error of modernism identified by the belief that: The Church ought to be separated from the State, and the State from the Church. (Papal Allocution, Acerbissimum, 27 September 1852).
An error of modernism identified by the belief that: 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. (Papal Allocution, Nemo Vestrum, 26 July 1855).
And, since where religion has been removed from civil society, and the doctrine and authority of divine revelation repudiated, the genuine notion itself of justice and human right is darkened and lost, and the place of true justice and legitimate right is supplied by material force, thence it appears why it is that some, utterly neglecting and disregarding the surest principles of sound reason, dare to proclaim that "the people's will, manifested by what is called public opinion or in some other way, constitutes a supreme law, free from all divine and human control; and that in the political order accomplished facts, from the very circumstance that they are accomplished, have the force of right." But who, does not see and clearly perceive that human society, when set loose from the bonds of religion and true justice, can have, in truth, no other end than the purpose of obtaining and amassing wealth, and that (society under such circumstances) follows no other law in its actions, except the unchastened desire of ministering to its own pleasure and interests? (Papal Encyclical, Condemning Current Errors: Quanta Cura, 8 December 1986).