This kingdom is to endure for ever, gradually to embrace all the inhabitants of the earth, and finally the entire moral government of God in heaven and on earth. The little stone which breaks the image will become a great mountain and fill the whole earth (Dan. 2:35). This gospel of the kingdom is to be preached to all nations. Then all the kingdoms of this world shall become the kingdoms of our Lord and of his Christ, and he shall reign for ever and ever. And in the dispensation of the fullness of times all things, both which are in heaven and which are on earth, are to be gathered together in one in Christ; who is set at the right hand of God in heavenly places, far above all principality and power and might and dominion, and every name that is named, not only in this world, but also in that which is to come; and all things are put under his feet, He only being excepted that did put all things under him (Eph. 1:10, 20, 21; 1 Cor. 15:27).
III. The process by which this kingdom grows through its successive stages toward its ultimate completion can of course be very inadequately understood by us. It implies the ceaseless operation of the mighty power of God working through all the forces and laws of nature and culminating in the supernatural manifestations of grace and of miracle. The Holy Ghost is everywhere present, and he works directly alike in the ways we distinguish as natural and as supernatural—alike through appointed instruments and agencies, and immediately by his direct personal power. The special agency for the building up of this kingdom is the organized Christian Church with its regular ministry, providing for the preaching of the gospel and the administration of the sacraments. The special work of the Holy Ghost in building up this kingdom is performed in the regeneration and sanctification of individuals through the ministry of the Church. But beyond this the omnipresent Holy Ghost works to the same end, directly and indirectly, in every sphere of nature and of human life, causing all the historic movements of peoples and nations, of civilization and of science, of political and ecclesiastical societies, to broaden and deepen the foundations and to advance the growth and perfection of his kingdom. Thus this kingdom from the beginning and in the whole circle of human history has been always coming. Its coming has been marked by great epochs, when new revelations and new communications of divine power have been imported from without into the current of human history. The chiefest of these have been the giving of the law, the incarnation, crucifixion, resurrection, ascension and session of the King on the right hand of the Father, and the mission of the Holy Ghost. Yet the kingdom has been always coming every moment of all the years that have passed. In all the growing of the seeds and all the blowing of the winds; in every event, even the least significant, which has advanced the interests of the human family either in respect to their bodies or their souls, and thus made their lives better or worthier; in all the breaking of fetters; in all the bringing in of light; in the noiseless triumphs of peace; in the dying out of barbarisms; and in the colonization of great continents with new populations and free states,—the kingdom is coming. Above all, in the multiplication of the myriad centres of Christian missions and of the myriad hosts of Christian workers, each in the spirit of the King seeking the very lowest and most degraded, everywhere lifting upward what Satan's kingdom has borne down,—the kingdom is coming. Its process is like that of the constructive power of the kingdom of nature, silent and invisible, yet omnipresent and omnipotent, like the rain and the dew and the zephyr and the sunlight. The kingdom comes intensively in each heart like the leaven, which penetrates the whole mass silently yet irresistibly until all is leavened. It comes extensively like the growth of the mustard-seed, which from the least beginnings unfolds itself until it shoots out great branches and shelters the fowls of heaven. In this World the wheat and the tares, the good and the evil, grow together to the end. The net gathers in fish good and bad. One field brings forth thirty, another forty, and another an hundred-fold. In the end the tares shall be gathered and burned, and the pure wheat gathered without mixture in the eternal garner of the Lord. In the whole history of its coming the kingdom of God "cometh not with observation: neither shall they say, Lo, here! or, Lo, there! for behold, the kingdom of God is within you." But its consummation shall be ushered in suddenly and with overwhelming demonstrations of glory: "For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in his day." For the present the King is absent, gathering together in his grasp the reins of his empire: we are left to be diligently employed with the doing the utmost for his cause possible within our respective spheres against his coming. When he comes he will be revealed as a King of kings, followed by great retinues of royal princes sitting on thrones and reigning over cities in his name and through his grace.
-- A. A. Hodge, Evangelical Theology, p. 255-8.