The truth that Christ is King is very old; the feast of Christ the King is very new.
The truth is old. In the Old Testament, God promises King David that he will have a descendant who will be a great king with a throne that last forever; a king who will build a house to God's name; a kingdom where God will be a father to the King. The prophets of the Old Testament pointed to a future messiah who would rule over Israel. The Jews believed that the messiah would indeed be a king. The kings of Israel were anointed with oil; "messiah" means "anointed." This was all fulfilled in Christ, who, in His human origins, is Son of David.
In the New Testament, Saint John has a vision of Christ in great splendor and power, and on Christ's garment and thigh are written : King of Kings and Lord of Lords. (Revelations 19:15)
The Church has always regarded Christ as King. But the feast was only put on the Church calendar in 1925. Why?
What was happening in the world in that Jubilee Year of 1925 and the years before it? It was a time of great social and political tension (as in every age). Europe was getting rid of many of its kings. It was also the aftermath of the First World War, one of the worst in history in terms of human casualties. The Communists were in power in Russia and communist/socialist parties in other countries were becoming powerful. Social classes were at odds; the workers versus the corporate bosses. Fascism won in Italy and was starting in other nations.
For the Church, anti-clericalism was on the rise. Communist Russia was a sworn enemy of all religion. Even in traditionally Catholic countries, the Church was persecuted because it was accused of meddling in politics.
Pope Pius XI therefore proclaimed this new feast to remind the world of a couple of things. The first is that, no matter how many kings the world topples, one king will always remain : Christ the King. The Pope wrote that Christ is not just King over the Catholics of the world; Christ is King of the universe - period. Christ's authority is not exercised with guns, but with the proclamation of the truth; and the Church has the right to preach that truth and work for its implementation.
Secondly, the Pope taught that only a world obedient to Christ the King can bring true peace and security to the world. Christ is not King of the Jews; He is King of all who listen to the Truth, because He is the way, the truth and the life. His rule is a rule of mercy and justice, and if we were all under this rule, mercy and justice would put away this world's violence and injustice.
WHY IS CHRIST - AND NOT THE FATHER - KING?
"The Father is greater than I." (John 14:28). But Jesus is God (John 20:28), so how can one divine person (the Son) be less than another divine person (the Father)? Are they not equal? Yes, they are equal in divinity. As we say in the Creed, Christ is consubstantial with the Father (one in essence). As far as the divinity is concerned, Christ is equal to the Father. But the Son and the Father are not the same persons; the Son is begotten of the Father. The Son is the self-knowledge of the Father, and so He is called the Logos, the Word of God. Christ is not begotten in time, as if there was a time when He was not, and then, in time, He became. No; since He is God and therefore eternal, Christ is begotten in eternity (not in time); there was never a moment when the Word/Son was not. He always was, but He is begotten of the Father. Therefore, in this sense, the Father is greater than the Son.
Because He is the Word of God, the Father creates all things through the Word. God says (word) : Let there be light - and there is light! Everything created - angels - animals - humans - are created through the Word. In this sense, Christ is the King of all creation.
This creation was in need of redempton. Christ took on the nature of a creature (He became man), and redeemed us. In this second sense, Christ is King of redeemed creation. Only the angels in heaven (who do not need redemption), and the demons in hell (who are incapable of redemption) were not redeemed. Yet Christ is still their King because they were created through the Word (Christ did not create demons; He created angels, some of whom became demons by their own freely chosen rebellion).
Because He is King of the redeemed, Christ is King of the Church, the body of redeemed, of which He is the Head.
Thus, in Christ, one can see the unity and reconciliation of all things. God and creation were at odds since the Fall. In Christ, creation is united to a divine person. Through His death and resurrection, creation is redeemed and exalted. Thus, Christ has to be King; He has to be the leader who obeyed the Father to the point of shedding His blood, who brings rebellious man under the rule of God. He does not force man to submit; He redeems only those who are open to the Truth (see Gospel). Truth is the weapon of this King.
God the Father makes His Son King, because the Father sees in His obedient, loyal Son, the only way all creation can come back to the Father. "No one comes to the Father except through me." (John 14:6)
CHANGE OF THE FEAST DAY
Pope Pius XI decreed that this feast be celebrated on the last Sunday of October, so that it comes before the feast of All Saints. He saw the last Sunday of October as coming towards the end of the liturgical year, which had celebrated many of the more important events of the life and ministry of the Lord on the prior Sundays. This Sunday, therefore, was a kind of crowning of all those events, and precedes the celebration of the saints who are the fruit of Christ's redemption.
He also ordered that Catholics renew their consecration to the Sacred Heart on this feast.
Pope Paul VI moved this feast to the last Sunday of the liturgical year, whatever Sunday that may be on the civil calendar. It will always be the Sunday before the first Sunday of Advent. The whole previous liturgical year, therefore, would be crowned by the Kingship of Christ.
Since the Motu Proprio, under which we observe the traditional Mass (Extraordinary Form), decrees that we follow the rules and calendar of the 1962 Missal, the Extraordinary Form celebrates this feast on the last Sunday of October.
CHRISTUS VINCIT! CHRISTUS REGNAT! CHRISTUS IMPERAT!
Christ conquers! Christ reigns! Christ rules!