13 There follows in the prayer, Your kingdom come. We ask that the kingdom of God may be set forth to us, even as we also ask that His name may be sanctified in us. For when does God not reign, or when does that begin with Him which both always has been, and never ceases to be? We pray that our kingdom, which has been promised us by God, may come, which was acquired by the blood and passion of Christ; that we who first are His subjects in the world, may hereafter reign with Christ when He reigns, as He Himself promises and says, “Come, you blessed of my Father, receive the kingdom which has been prepared for you from the beginning of the world.” Christ Himself, dearest brethren, however, may be the kingdom of God, whom we day by day desire to come, whose advent we crave to be quickly manifested to us.
For since He is Himself the Resurrection, since in Him we rise again, so also the kingdom of God may be understood to be Himself, since in Him we shall reign. But we do well in seeking the kingdom of God, that is, the heavenly kingdom, because there is also an earthly kingdom. But he who has already renounced the world, is moreover greater than its honours and its kingdom. And therefore he who dedicates himself to God and Christ, desires not earthly, but heavenly kingdoms. But there is need of continual prayer and supplication, that we fall not away from the heavenly kingdom, as the Jews, to whom this promise had first been given, fell away; even as the Lord sets forth and proves: “Many,” says He, “shall come from the east and from the west, and shall recline with Abraham, and Isaac, and Jacob in the kingdom of heaven.
But the children of the kingdom shall be cast out into outer darkness: there shall be weeping and gnashing of teeth.” He shows that the Jews were previously children of the kingdom, so long as they continued also to be children of God; but after the name of Father ceased to be recognised among them, the kingdom also ceased; and therefore we Christians, who in our prayer begin to call God our Father, pray also that God's kingdom may come to us.
Source: The Treatises of Cyprian (New Advent)