28 A notable difference exists between every kind of civil rule and that of the kingdom of Christ. If this latter bear a certain likeness and character to a civil kingdom, it is distinguished from it by its origin, principle, and essence. The Church, therefore, possesses the right to exist and to protect herself by institutions and laws in accordance with her nature. And since she not only is a perfect society in herself, but superior to every other society of human growth, she resolutely refuses, promoted alike by right and by duty, to link herself to any mere party and to subject herself to the fleeting exigencies of politics. On like grounds, the Church, the guardian always of her own right and most observant of that of others, holds that it is not her province to decide which is the best amongst many diverse forms of government and the civil institutions of Christian States, and amid the various kinds of State rule she does not disapprove of any, provided the respect due to religion and the observance of good morals be upheld. By such standard of conduct should the thoughts and mode of acting of every Catholic be directed.
Source: Sapientiae Christianae (Vatican.va)