Pope Pius XI instituted this feast in 1925 with his encyclical Quas Primas ("In the first"). . . to respond to growing nationalism and secularism. He recognized that these related societal ills would breed increasing hostility against the Church. His encyclical reminds the faithful that while governments and philosophies come and go, Christ reigns as king forever.
The kingdom, already present and growing in our midst, engages us at every level of our being and reminds us of the principle of discernment which Pope Paul VI applied to true development: it must be directed to 'all men and the whole man'.
– Pope Francis, Evangelii gaudium, 181