From Pope Pius XI’s encyclical Quas Primas (1925):
When we pay honor to the princely dignity of Christ, men will doubtless be reminded that the Church, founded by Christ as a perfect society, has a natural and inalienable right to perfect freedom and immunity from the power of the state; and that in fulfilling the task committed to her by God of teaching, ruling, and guiding to eternal bliss those who belong to the kingdom of Christ, she cannot be subject to any external power. The State is bound to extend similar freedom to the orders and communities of religious of either sex, who give most valuable help to the Bishops of the Church by laboring for the extension and the establishment of the kingdom of Christ. By their sacred vows they fight against the threefold concupiscence of the world; by making profession of a more perfect life they render the holiness which her divine Founder willed should be a mark and characteristic of his Church more striking and more conspicuous in the eyes of all.
Nations will be reminded by the annual celebration of this feast that not only private individuals but also rulers and princes are bound to give public honor and obedience to Christ. It will call to their minds the thought of the last judgment, wherein Christ, who has been cast out of public life, despised, neglected and ignored, will most severely avenge these insults; for his kingly dignity demands that the State should take account of the commandments of God and of Christian principles, both in making laws and in administering justice, and also in providing for the young a sound moral education.
— Quas Primas 31-32