The angel Gabriel to the Virgin Mary: "The Lord God shall give unto him the throne of his father David. And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end."
The Magi to Herod: "Where is he that is born king of the Jews?"
Charles Wesley to the church: "Hark! the herald angels sing, 'Glory to the newborn king.'"
The kingship of Israel's Messiah is deeply ingrained in the stories and songs of Christmas. Yet, in our modern and post-modern world, we don't really relate to royalty (other than to gossip about princes cavorting in Las Vegas). Royalty isn't much of a category for us.
In the early 20th century, however, the western world was in turmoil over the best form of governance. In Russia in 1917, political pressures led Tsar Nicholas II to abdicate. The Bolshevik revolution threw the country into anarchy, and the royal family was executed the following year. In Mexico, a 1917 constitution was antagonistic not only to monarchs, but to the Catholic church as well. That Mexican constitution served as a model for the new Russian Constitution of 1918 and the Weimar Republic's German constitution of 1919. In Spain, a 1923 military coup undermined the monarch's power. In Italy, the Kingdom of Italy invaded the Papal States in 1860 and Rome in 1870. The next six decades saw hostile relations between the government and the papacy. Mussolini's fascists imposed martial law in 1922, assassinated opposition politicians in 1924, and by 1925 dropped all pretense of democracy.
Christ the King in 1925
In the middle of this anti-monarchist, anti-Catholic ferment, Pope Pius XI sat down to write an Encyclical Letter (Quas Primas) on the kingship of Christ. The letter was released in December 1925, when the Pope still considered himself a prisoner in the Vatican. It would be over three years before a treaty would grant the Pope and Vatican City a stripped-down but legitimate political status. Pius was fighting for his political life, and he articulated a theology of Christ's kingship as a means to sway opinion in his favor. Beyond theology, Pius gave the faithful a new liturgical feast: The Feast of Christ the King.
This feast has been adopted by a number of Protestant denominations—Lutheran, Anglican, Presbyterian, Methodist—as a result of the liturgical renewal of the 1960s.
Pius noted that it is common to call Christ "king" metaphorically, because he reigns in people's hearts and wills. But Pius's purpose was to outline a kingship that was more than metaphorical and that extended over more than the personal, interior realm of the believer. He believed that Christ's kingship "invests the human authority of princes and rulers with a religious significance; it ennobles the citizen's duty of obedience." Thus, Pius hoped to reconnect the this-worldly offices of king and prince with the divine rulership of Christ himself. This was a throwback to medieval ways of thinking.
In addition, he hoped to assert the freedom of the church from any domination by the state. He wrote 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." Most Protestants agree, but for a more individualistic reason. We want freedom to follow our own beliefs, and we are usually willing to grant the same freedom to other religions and denominations as long as they don't interfere with ours.