A still valid principle
 
The doctrine of the social reign of Christ 
 
This doctrine belongs to the tradition of the Church and, as such, is also valid today and will always be 
 
The principal doctrinal of the "social reign of Christ" means that the construction of human society will not be able to achieve its own natural ends without being ordained to Jesus Christ, Creator, and Savior. He, as a Creator, has constituted human society based on marriage and family, on reciprocal love and authority. Always as a Creator, the Lord has given society the rules of coexistence, setting in everything its limits. As a Redeemer, the Lord Jesus has re-created for the second time the world after sin and in the end will recap all things in himself, both heaven and earth. Jesus Christ has absolute dominion over history and the world because He is the Alpha and the Omega. Also, as Joseph Ratzinger wrote, "A God who has no power is a contradiction in terms." In memory and identity, John Paul II wrote that Christ has a real mission: "He is subjected to all things until He submits to the Father along with everything created for God to be everything in everything." 
 
A principle still valid 
 
The doctrine of the social reign of Christ was established and taught by Pius XI in the encyclical Quas bonuses of 1925, but already the previous pontiffs had expressed their meaning, as Leo XIII in the encyclical Immortal Dei. That is why we say that this doctrine belongs to the tradition of the Church and, as such, is also valid today and will always be. Unfortunately, a wall has often been erected between the social doctrine of the reconcile and the postreconcile Church. So you might think that this doctrine, expressed at that time, is no longer valid today. But it's not like that. 
 
The constitution Gaudium et spes of the Second Vatican Council affirms that "The creature without the Creator disappears" (N. 36). The constitution Lumen Gentium says that the laity must "order the temporal affairs according to God." The Apostolicam decree actuositatem teaches that it is for the laity to "fill in a Christian spirit the thought and the customs, the laws, and the structures of the community in which one lives" (N. 13). All these are undoubted references to the reign of Christ. 
 
The Catechism of the Catholic Church dedicates to this topic especially paragraph 2105, where it reiterates "the royalty of Christ on all creation and, in particular, on human societies".
 
John Paul II enunciated this doctrine from the first moment, in the homily of his first mass as pontiff: "Do not fear! Open, even more, open wide the doors to Christ! Open up to his saving power the confines of States, the economic systems and politicians, the vast fields of culture, of civilization and development. Don't be afraid! Christ knows "what is an inside man." Only He knows! " Notice that here the Holy Pope invited us to open to Christ not only the hearts but also the economic and political systems, whose construction is not indifferent to the Lord. 
Benedict XVI has repeatedly underlined the concept of Christ's royalty: "with the extinguishment of the light that comes from God, humanity is affected by the lack of guidance, whose destructive effects are becoming more and more of manifest", (March 10, 2009). He has also done it on January 19, 2012, with a lapidary phrase: "there is not a kingdom of earthly issues that can substract the Creator and his dominion". 
Pope Francis has written in the Evangelii Gaudium: "it is not the same to have known Jesus that not to know him, it is not the same to walk with him that to grope, it is not the same to be able to listen to him that to ignore his Word, it is not the same to contemplate , to adore him, to rest in him, that not to be able to do. It is not the same to try to build the world with his Gospel than to do it only with one's reason "(N. 266).
 
On it is based the social doctrine of the Church 
 
It should not be forgotten that the principle of the reign of Christ is based on the social doctrine of the Church. Why? Because it was born the social doctrine of the Church in its modern form. Benedict XVI said that the need for the new evangelization goes back to the eighteenth century when States wanted to eliminate God from public space. Well, the [modern] social Doctrine of the Church begins there, especially with Leo XIII, to put God at the center of the construction of society and politics. Indeed, Leo XIII, in Rerum Novarum, wrote that the social question is "a problem whose acceptable solution would be truly null if it were not sought under the auspices of religion and the Church" (N. 12). This conviction is not obsolete, but is also valid today, so much so that one hundred years later, in the Centesimus annus, John Paul II confirms this teaching: "As then, it must be repeated that there is no real solution for the "social question" outside the Gospel" (n. 5). Only the reference to Christ saves society and allows us to identify and pursue the Common Good. And this is nothing other than the doctrine of Christ's social reign. 
 
This is so important for the social Doctrine of the Church that, if we eliminate the social doctrine of the reign of Christ, it would be transformed into social ethics, a list of good intentions, in a handbook of good practices. But the High Pontiffs have never understood the social Doctrine of the Church in this way. John Paul II said that "the social doctrine has in itself the value of an instrument of evangelization: as such, it announces to God and his mystery of salvation in Christ to every man and, for the same reason, reveals to the man himself. Only under this perspective does it deal with the rest "(Centesimus annus, N. 54). 
 
The social reign of Christ is an expression of the Christian claim to proclaim salvation in Christ. Otherwise, the social Doctrine of the Church would be a sweetener for the ills of society or a balm for the wounds of injustices, Christ is not only useful, But He is also indispensable. It is said by the Caritas in Veritate of Benedict XVI: "adherence to the values of Christianity is not only a useful element but indispensable for the construction of a good society and a true integral human development" (N. 4). How could God be only useful and not indispensable? And how could He be indispensable, without expressing royalty over temporal things? That is why the declaration Dignitatis Humanae of Vatican II states that "there is a moral duty of men and societies to the true religion and the only Church of Christ" (N. 1).