From Pope Pius XI’s encyclical Quas Primas (1925):
For the fulfillment of the plan of which We have spoken, the Holy Year, which is now speeding to its close, offers the best possible opportunity. For during this year the God of mercy has raised the minds and hearts of the faithful to the consideration of heavenly blessings which are above all understanding, has either restored them once more to his grace, or inciting them anew to strive for higher gifts, has set their feet more firmly in the path of righteousness. Whether, therefore, We consider the many prayers that have been addressed to Us, or look to the events of the Jubilee Year, just past, We have every reason to think that the desired moment has at length arrived for enjoining that Christ be venerated by a special feast as King of all mankind. In this year, as We said at the beginning of this Letter, the Divine King, truly wonderful in all his works, has been gloriously magnified, for another company of his soldiers has been added to the list of saints. In this year men have looked upon strange things and strange labors, from which they have understood and admired the victories won by missionaries in the work of spreading his kingdom. In this year, by solemnly celebrating the centenary of the Council of Nicæa. We have commemorated the definition of the divinity of the word Incarnate, the foundation of Christ’s empire over all men.
— Quas Primas 27