192. If authority belongs to the order established by God, the choice of the political regime and the appointment of rulers are left to the free decision of the citizens (GS, n. 74). The diversity of political regimes is morally acceptable, provided they serve the legitimate good of the communities that adopt them. Regimes whose nature is contrary to the natural law, to the public order, and to the fundamental rights of persons cannot achieve the common good of the nations on which they have been imposed.
(CCC, n. 1901)

193. In modern times, this concept has been opposed by totalitarianism, which, in its Marxist Leninist form, maintains that some
people, by virtue of a deeper knowledge of the laws of the development of society, or through membership of a particular class or through contact with the deeper sources of the collective consciousness, are exempt from error and can therefore arrogate to themselves the exercise of absolute power. It must be added that totalitarianism arises out of a denial of truth in the objective sense. If there is no transcendent truth, in obedience to which man achieves his full identity, then there is no sure principle for guaranteeing just relations between people. Their self interest as a class, group or nation would inevitably set them in opposition to one another. If one does not acknowledge transcendent truth, then the force of power takes over, and each person tends to make full use of the means at his disposal in order to impose his own interests or his own opinion, with no regard for the rights of others. People are then respected only to the extent that they can be exploited for selfish ends. Thus, the root of modern totalitarianism is to be found in the denial of the transcendent dignity of the human person who, as the visible image of the invisible God, is therefore by his very nature the subject of rights which no one may violate no individual, group, class, nation or State. Not even the majority of a social body may violate these rights, by going against the minority, by isolating, oppressing, or exploiting it, or by attempting to annihi late it. The culture and praxis of totalitarianism also involve a rejection of the Church. The State or the party which claims to be able to lead history towards perfect goodness, and which sets itself above all values, cannot tolerate the affirmation of an objective criterion of good and evil beyond the will of those in power, since such a criterion, in given circumstances, could be used to judge their actions. This explains why totalitarianism attempts to destroy the Church, or at least to reduce her to submission, making her an instrument of its own ideological apparatus. Furthermore, the totalitarian State tends to absorb within itself the nation, society, the family, religious groups and individuals them selves. In defending her own freedom, the Church is also defending the human person, who must obey God rather than men (cf. Acts5:29), as well as defending the family, the various social organizations and nations all of which enjoy their own spheres of autonomy and sovereignty.
(Centesimus Annus, nn. 44 45)

194. In determining the structure and operation of government which a State is to have, great weight has to be given to the circumstances of a given people, circumstances which will vary at different times and in different places. We consider, however, that it is in keeping with the innate demands of human nature that the State should take a form which embodies the three fold division of powers corresponding to the three principal functions of public authority. In that type of State, not only the official functions of government but also the mutual relations between citizens and public officials are set down according to law, which in itself affords protection to the citizens both in the enjoyment of their rights and in the fulfillment of their duties.
(Pacem in Terris,n. 68)

195. If the citizens' responsible cooperation is to produce the good results which may be expected in the normal course of political life, there must be a statute of positive law providing for a suitable division of the functions and bodies of authority and an efficient and independent system for the protection of rights. The rights of all persons, families and groups, and their practical application, must be recognized, respected and furthered, together with the duties binding on all citizens. Among the latter, it will be well to recall the duty of rendering the political community such material and personal service as are required by the common good. Rulers must be careful not to hamper the development of family, social or cultural groups, nor that of intermediate bodies or organizations, and not to deprive them of opportunities for legitimate and constructive activity; they should willingly seek, rather, to promote the orderly pursuit of such activity. Citizens, for their part, either individually or collectively, must be careful not to attribute excessive power to public authority, not to make exaggerated and untimely demands upon it in their own interests, lessening in this way the responsible role of persons, families and social groups.
(Gaudium et Spes, n. 75)

196. When we speak of the reform of institutions, the State comes chiefly to mind, not as if universal well being were to be expected from its activity, but because things have come to such a pass through the evil of what we have termed `individualism' that, following upon the overthrow and near extinction of that rich social life which was once highly developed through associations of various kinds, there remain virtually only individuals and the State. This is to the great harm of the State itself, for, with a structure of social governance lost, and with the taking over of all the burdens which the wrecked associations once bore, the State has been overwhelmed and crushed by almost infinite tasks and duties.
(Quadragesimo Anno, n. 78)