What is Christ's Kingdom?

The kingdom of God, the kingdom of heaven, and the kingdom of Christ all refer to one kingdom. Its source is God in heaven - it is not "from the world" (John 18:36). Christ is the king of this kingdom in his capacity as mediator. Since He is God, Jesus has always had universal dominion. But as Matthew 28:18 says, "All authority in heaven and on earth has been given" to Jesus as our redeemer on the basis of His death and resurrection for the purpose of putting this sinful world into subjection to God. He does this by subduing some by grace, making them willing citizens of the kingdom, and by subduing others by judgment. Psalms 2 and 110, as well as Revelation 5, 1 Corinthians 15:20-28, and Colossians 1:15-20, explain this glorious truth. As the Westminster Shorter Catechism puts it: "Christ executeth the office of a king, in subduing us to himself, in ruling and defending us, and in restraining and conquering all his and our enemies."

In a special manner, the church is the institutional expression of the kingdom of Christ. The "keys of the kingdom" were given to the church (Matt. 16:16-19) and only by being born again by water and the Spirit can one enter the kingdom (John 3:5). Christians are the citizens of the kingdom. The kingdom can also be thought of more broadly as the reign of Christ. As the Spirit brings people to believe and obey Jesus, these are manifestations of the kingdom. As Paul says, the kingdom is a matter of "righteousness and peace and joy in the Holy Spirit" (Rom. 14:17). This Spirit works primarily by the Word of God (Eph. 6:17). Thus we advance Christ's kingdom primarily by proclaiming God's Word, as well as the right use of the sacraments, prayer, etc., relying on the Spirit to apply Christ's redemption and manifesting the fruits of the Spirit in everything we do. Furthermore, the claims of Christ's kingdom are universal, extending to all authorities on heaven and earth (Matt. 28:18), even civil leaders (Ps. 2). As Louis Berkhof says,
"The visible Church may certainly be said to belong to the Kingdom, to be a part of the Kingdom, and even to be the most important visible embodiment of the forces of the Kingdom…In so far as the visible Church is instrumental in the establishment and extension of the Kingdom, it is, of course, subordinate to this as a means to an end. The Kingdom may be said to be a broader concept than the Church, because it aims at nothing less than the complete control of all the manifestations of life. It represents the dominion of God in every sphere of human endeavor." (Systematic Theology, p. 570)
As for its extent and growth, look at the dream and interpretation of Nebuchadnezzar in Daniel 2:31-45, especially with regard to the rock. The rock is the kingdom that God will set up in the days of the kings of the iron and clay (i.e. the emperors of Rome). It will shatter all opposition and will fill the earth. It will never be destroyed. The New Testament parables also tell of this irresistible growth and eventual universal impact (i.e. Matt. 13:31-33). And in the parables of the wheat and weeds (i.e. wheat and tares) in Matthew 13:24-30, 36-43, the field is the world at the beginning (vs. 38) and the wheat that is sown is the sons of the kingdom (vs. 38). The weeds will mix with the kingdom until the final judgement when the kingdom will be completely refined (13:41-43). But by the end, the field (that the wheat and weeds are taken from) is already the kingdom (13:41).

And so we pray, as Christ taught us, "Thy kingdom come...on earth as it is in heaven." The Westminster Larger Catechism summarizes what this prayer means in the context of the whole Bible by saying:

"In the second petition, (which is, Thy kingdom come,) acknowledging ourselves and all mankind to be by nature under the dominion of sin and Satan, we pray, that the kingdom of sin and Satan may be destroyed, the gospel propagated throughout the world, the Jews called, the fullness of the Gentiles brought in; the church furnished with all gospel-officers and ordinances, purged from corruption, countenanced and maintained by the civil magistrate: that the ordinances of Christ may be purely dispensed, and made effectual to the converting of those that are yet in their sins, and the confirming, comforting, and building up of those that are already converted: that Christ would rule in our hearts here, and hasten the time of his second coming, and our reigning with him forever: and that he would be pleased so to exercise the kingdom of his power in all the world, as may best conduce to these ends." (Westminster Larger Catechism, Q. 191)

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If you are interested in reading more, here are some of my blog posts on this topic:

The Kingship of Christ
John Calvin on Christ's Kingdom
Living as Citizens of Heaven
A.A. Hodge on the Universal Reign of Christ