From Pope Pius XI’s encyclical Quas Primas (1925):

It was surely right, then, in view of the common teaching of the sacred books, that the Catholic Church, which is the kingdom of Christ on earth, destined to be spread among all men and all nations, should with every token of veneration salute her Author and Founder in her annual liturgy as King and Lord, and as King of Kings. And, in fact, she used these titles, giving expression with wonderful variety of language to one and the same concept, both in ancient psalmody and in the Sacramentaries. She uses them daily now in the prayers publicly offered to God, and in offering the Immaculate Victim. The perfect harmony of the Eastern liturgies with our own in this continual praise of Christ the King shows once more the truth of the axiom: Legem credendi lex statuit supplicandi. The rule of faith is indicated by the law of our worship.

— Quas Primas 12