In Pius XI's encyclical Quas Primas, we read:
What We said at the beginning of Our Pontificate concerning the decline of public authority, and the lack of respect for the same, is equally true at the present day. "With God and Jesus Christ," we said, "excluded from political life, with authority derived not from God but from man, the very basis of that authority has been taken away, because the chief reason of the distinction between ruler and subject has been eliminated. The result is that human society is tottering to its fall, because it has no longer a secure and solid foundation."...If princes and magistrates duly elected are filled with the persuasion that they rule, not by their own right, but by the mandate and in the place of the Divine King, they will exercise their authority piously and wisely, and they will make laws and administer them, having in view the common good and also the human dignity of their subjects.
....When once men recognize, both in private and in public life, that Christ is King... Men will see in their king or in their rulers men like themselves, perhaps unworthy or open to criticism, but they will not on that account refuse obedience if they see reflected in them the authority of Christ God and Man.
1897 "Human society can be neither well-ordered nor prosperous unless it has some people invested with legitimate authority to preserve its institutions and to devote themselves as far as is necessary to work and care for the good of all." By "authority" one means the quality by virtue of which persons or institutions make laws and give orders to men and expect obedience from them.
1898 Every human community needs an authority to govern it. The foundation of such authority lies in human nature. It is necessary for the unity of the state. Its role is to ensure as far as possible the common good of the society.
1899 The authority required by the moral order derives from God: "Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. Therefore he who resists the authorities resists what God has appointed, and those who resist will incur judgment."
I have added the underlinings in both quotations, as it seems to represent the source of a particular integrist interpretation of the teaching of Quas Primas that is not reflected in the teaching of the Catechism or the section of Gaudium et Spes (nn.73-76) on which the Catechism draws. References for the citations in the extract from the Catechism can be found by following the link to the Catechism above..
The idea that all authority has its (direct) origin in God and, from a Christian point of view, in the person of Christ to whom the Father has entrusted all things, is what gives rise to the notion that that authority can only be rightly exercised in a confessional Catholic state. This contrasts with the suggestion of the Catechism that political authority has a foundation in human nature, distinguished from an authority in the moral order which derives from God. It also contrasts with the teaching of n.76 of Gaudium et Spes that:
The Church and the political community in their own fields are autonomous and independent from each other.
The resolution of this contrast is found in a subtle formulation of Gaudium et Spes, to which the Catechism refers, and emphasised again by my added italics:
Yet the people who come together in the political community are many and diverse, and they have every right to prefer divergent solutions. If the political community is not to be torn apart while everyone follows his own opinion, there must be an authority to direct the energies of all citizens toward the common good, not in a mechanical or despotic fashion, but by acting above all as a moral force which appeals to each one's freedom and sense of responsibility.
It is clear, therefore, that the political community and public authority are founded on human nature and hence belong to the order designed by God, even though the choice of a political regime and the appointment of rulers are left to the free will of citizens.
Where Pius XI uses the language of direct representation of the authority of God in the exercise of authority in public office, Gaudium et Spes instead speaks of a public authority that is exercised within a created order originating from God. The former might be seen as an expression in the order of revelation, while the latter is a like expression in the order of creation.
It is worth appreciating three aspects of context for Pius XI's remarks about the nature of authority in political life. Firstly, Quas Primas has its immediate purpose in establishing, as a celebration in the universal Church, of the liturgical feast of Christ the King, and therefore the relevance of Christ's kingship to the religious life of Catholics. Secondly, the encyclical also has a section that surveys the different places where the Old and New Testaments refer to Christ as a King, or as one who will rule. And thirdly, the discussion of the relationship between the kingship of Christ and authority exercised by civil powers is offered in response to Pius XI's perception of a rejection of Christ by rulers and states of his time (perhaps with a reference to the rise of fascism in Italy and atheistic communism in Russia?).
Though Quas Primas urges that rulers and states should acknowledge in their public life the rule of Christ, this has more the sense of an impulse for evangelisation than an advocacy in favour of a confessional Catholic state. This is particularly seen to be the case if the discussion of civil authority is placed alongside such sections of Quas Primas as the survey of Scriptural texts, which present Christ's kingship in a messianic and eschatological perspective, and its discussion of the feast day in relation to the lives of Catholics themselves.
The penultimate paragraph of Quas Primas provides a useful reflection as we approach the feast of Christ the King this year:
If to Christ our Lord is given all power in heaven and on earth; if all men, purchased by his precious blood, are by a new right subjected to his dominion; if this power embraces all men, it must be clear that not one of our faculties is exempt from his empire. He must reign in our minds, which should assent with perfect submission and firm belief to revealed truths and to the doctrines of Christ. He must reign in our wills, which should obey the laws and precepts of God. He must reign in our hearts, which should spurn natural desires and love God above all things, and cleave to him alone. He must reign in our bodies and in our members, which should serve as instruments for the interior sanctification of our souls, or to use the words of the Apostle Paul, as instruments of justice unto God.