The Covenantal Kingdom
The Biblical and Theological Issues
Biblical Answer: The Victory of the Gospel (Part One)
We begin with the second question: Will the church age be a time of evident prosperity for the Gospel on earth? The postmillennialist agrees with the premillennialist that the many promises in the Old Testament of an age of great blessing on earth will be fulfilled. Both sides also agree that the promises will be fulfilled at a time when men are evidently still in mortal bodies, for sin and death are still facts of life, even when human societies are enjoying the fullest measure of earthly blessing the Gospel will ever bring (Is. 65:20). 
In contrast with the premillennialist the postmillennialist believes that these blessings are brought about not by the return of Christ, but by the work of the Holy Spirit  through the Gospel in the church age. How can the postmillennialist demonstrate Biblically that the blessings of God's kingdom come through the spread of the Gospel before the second coming of Christ?
To begin with, the postmillennialist sees in the Great Commission a promise of success.  Consider the preface to the Great Commission. Jesus said: "All authority is given unto me in heaven and in earth" (Mt. 28:18). Note that Christ claims not only authority in heaven, but also all authority on earth. This is a clear assertion of His sovereignty over earth's history. It also means, of course, that Jesus' commission to His Church is backed by His own supreme and unimpeachable authority. Why did Jesus assure us that He has all authority in heaven and on earth, and then promise us that He Himself, the sovereign Lord, would be with us? Was it not to give us assurance that we should accomplish the task by His grace? No other interpretation does justice to the Biblical parallels or to the immediate context of Jesus' resurrection victory.
Next, consider the last words of Jesus' command to the Church: "[L]o, I am with you always, even unto the end of the world" (Mt. 28:20). Just as the Lord told Joshua that He would be with him and not leave or forsake him (Josh. 1:5-9), Christ has assured the Church that He will always be with her, even to the end of the age. Christians agree that God's promise to be with Joshua is a guarantee of his success. Why is it, then, that only postmillennialists believe Jesus' promise to the Church, that she will be enabled by His power to accomplish the great task to which He called her? If this is not the meaning of the promise of His presence, what does that promise mean?
But there is more. The promise of Christ's presence in the Great Commission is the fulfillment of Jesus' name as Matthew records it: "they shall call his name Immanuel, which being interpreted is, God with us" (Mt. 1:23; cf. Is. 7:14). Matthew records that Jesus is the One who fulfilled the Old Testament prophecies of the Messiah who is "God with us." The Immanuel promise in the Old Testament expresses the very essence of the covenant grace of God. God's presence assures the outward and eternal prosperity of His covenant people. It is God's presence that His people seek as the essence of covenantal blessing (cf. Ps. 27:4). Thus God promises Isaac: "Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father" (Gn. 26:3). God's presence guarantees His blessing.
When Jesus said, "[L]o, I am with you always, even unto the end of the world," He was pointing to the very meaning of His coming as Immanuel: that the presence of God assures the victory of His covenantally faithful people. 
Christians are to believe that His command carries with it the power for its accomplishment. As God said to Joshua, "Have I not commanded you? Be strong and of good courage; do not be afraid, nor be dismayed, for the LORD your God is with you wherever you go" (Josh. 1:9).  Christians must also "be strong and of good courage" because we know that God's promise and presence cannot fail. We must preach the Gospel with the confidence that God will prosper His Word and build His kingdom according to His covenant promise. With His presence the Church cannot fail!
Romans 11 is a central passage on the spread of the Gospel. It is not, as some hold, a discourse on the second coming of Christ. Paul outlines the progress of the Gospel in three general stages. First, Israel as a whole having rejected Christ, a remnant of the Jews and a large number of Gentiles are converted to faith in Christ. Second, God's evident blessing on Christian Gentiles eventually provokes the Jews to jealousy and becomes the means of leading them to faith in the Gospel.  Third, the conversion of Israel results in the salvation of the world. Paul says nothing here of Christ's return; he is only speaking of the growth and influence of the Gospel. According to Paul, the progress of the Gospel will bring about the salvation of the world. 
. Note Amillennialist Hoekema's unsuccessful attempt to interpret this passage in The Bible and the Future (Grand Rapids: Eerdmans, 1979), pp. 202 ff. Gary North demonstrates clearly that an amillennial interpretation of this passage is impossible. Millennialism and Social Theory (Tyler, Texas: Institute for Christian Economics, 1990), pp. 98-106.
. Note that postmillennialists believe that the kingdom is brought in by God, not man. It is the work of the Spirit through the Church. C. C. Ryrie falsely asserts that postmillennialism believes that the kingdom is brought in by the work of man: C. C. Ryrie, The Basis of the Premillennial Faith (Neptune, New Jersey: Loizeaux Brothers, 1953), pp. 13-14. Postmillennialists are not talking about a natural process or merely human effort, but about supernatural regeneration as the foundation of the kingdom. If the Spirit of God does not regenerate the world, there will be no kingdom of God in history -- premil or postmil.
. See: Kenneth J. Gentry Jr., "The Greatness of the Great Commission," in Gary North, ed., Journal of Christian Reconstruction, Vol 7, No. 2, Winter, 1981 (Vallecito, California: Chalcedon, 1981), pp. 19-47; and Gary DeMar and Peter Leithart, The Reduction of Christianity (Ft. Worth, Texas: Dominion Press, 1988), pp. 178-85. I have concentrated in the above paragraphs on the promise associated with the commission, but the commission itself implies the worldwide conquest of the Gospel also, inasmuch as we are commanded to "disciple all the nations." For a detailed book-length explanation of the Great Commission, see: Kenneth J. Gentry Jr., The Greatness of the Great Commission (Tyler, Texas: Institute for Christian Economics, 1990).
. See: Gn. 26:3, 24, 28; 28:15, 20; 31:3; 39:2, 3, 21, 23; 48:21; Ex. 3:12; 10:10; 18:19; 20:20; Nm. 14:9; 16:3; 23:21; Dt. 32:12; Josh. 1:5, 9, 17; 3:7; 6:27; 22:31; Jdg. 1:19, 22; 6:12, 13, 16; Rth. 2:4; 1 Sm. 3:19; 10:7; 14:7; 16:18; 17:37; 18:12, 14, 28; 20:13; 2 Sm. 7:3; 14:17; 1 Kg. 1:37; 8:57; 11:38; 2 Kg. 3:12; 10:15; 18:7; 1 Chr. 9:20; 17:2; 22:11; 16; 28:20; 2 Chr. 1:1; 13:12; 15:2, 9; 17:3; 19:11; 20:17; 36:23; Ezr. 1:3; Ps. 118:6, 7; Is. 8:10; 41:10; 43:2, 5; 45:14; Jer. 1:8, 19; 15:20; 20:11; 30:11; 42:11; 46:28; Zph. 3:17; Hag. 1:13; 2:4; Zch. 8:23; 10:5; and in the New Testament, cf. also: Mt. 1:23; Lk. 1:28; Acts 7:9; 10:38; 18:10; 2 Thes. 3:16; 2 Tim. 4:22; Rev. 21:3.
. Cf. Acts 13:47, "For so the Lord has commanded us: 'I have sent you to be a light to the Gentiles, that you should be for salvation to the ends of the earth.' "
. The reader will note that postmillennialists do believe in the fulfillment of the Old and New Testament promises that Israel will be saved. The mystery of the Gospel includes the idea, however, that the Gentiles and Jews will be one body in Christ.
. See the fuller discussion of the postmillennial order of development in Gary North, Unconditional Surrender: God's Program for Victory (Tyler, Texas: Dominion Press, third edition, 1988), pp. 335-47. On the future conversion of Israel in Romans 11, see: David Chilton, Paradise Restored: A Biblical Theology of Dominion (Ft. Worth, Texas: Dominion Press, 1985), pp. 125-31. For a detailed study of Romans 11 consult John Murray's commentary on Romans in the New International Commentary, and commentaries by Matthew Henry, Robert Haldane, and Charles Hodge.