The presence and coming of the Kingdom of God was the central message of Jesus. For example, "his teaching was designed to show men how they might enter the Kingdom of God (Matt. 5:20; 7:21). His mighty works
were intended to prove that the Kingdom of God had come upon them (Matt. 12: 28). His parables illustrated to His disciples the truth about the Kingdom of God (Matt. 13:11). And when He taught His followers to pray, at the heart of their petition were
the words, "Thy kingdom come, thy will be done on earth as it is in heaven" (Matt. 6:10). On the eve of His death, He assured His disciples that He would yet share with them the happiness and the fellowship of the Kingdom (Luke 22:22-30).
And He promised that He would appear again. on the earth in glory to bring the blessedness of the Kingdom to those for whom it was prepared (Matt. 25:31, 34)." 
The term "Kingdom of God" occurs four times in Matthew (12:28; 19:24; 21:31; 21:43), fourteen times in Mark, thirty-two times in Luke, twice in the
Gospel of John (3:3, 5), six times in Acts, eight times in Paul, and once in Revelation (12:10). Matthew actually prefers the term "Kingdom of heaven" which he
uses over 20 times in his gospel.
While Matthew primarily uses the term “kingdom of heaven” and other gospel writers (notably Luke) use the term “kingdom of God,” it is clear that these two expressions mean exactly the same thing (e.g. compare Matt. 5:3 with
Luke 6:20). In the past some have tried to maintain a distinction between the kingdom of heaven and the kingdom of God; however, the vast majority of theologians today recognize the terms as synonymous.
Graeme Goldsworthy has summarized a definition of the Kingdom of God as "God's people in God's place under God's rule."
Anthony Hoekema has described God's Kingdom as "the reign of God dynamically active in human history through Jesus Christ, the purpose of which is the redemption of his people from sin and from demonic powers, and
the final establishment of the new heavens and the new earth." 
George Eldon Ladd notes that "The primary meaning of both the Hebrew word malkuth in the Old Testament and of the Greek word basileia in the New Testament is the rank, authority and sovereignty exercised
by a king. A basileia may indeed be a realm over which a sovereign exercises his authority; and it may be the people who belong to that realm and over whom authority is exercised; but these are secondary and derived meanings. First of all,
a kingdom is the authority to rule, the sovereignty of the king." 
By George Eldon Ladd:
- Crucial Questions about the Kingdom of God. Eerdmans, 1952.
- Gospel of the Kingdom: Scriptural Studies in the Kingdom of God. Eerdmans, 1959.
- A Theology of the New Testament. Eerdmans, 1993, pp. 31-132.
- ↑ What is the Kingdom of God?, by George Eldon Ladd.
- ↑ For example, John F. Walvoord, Matthew - Thy Kingdom Come, (Moody Press, 1974) p. 30.
- ↑ For example, Ed Glasscock, Matthew, Moody Gospel Commentary (Moody Press, 1997), p. 70; D. A. Carson, Matthew, Expositor’s Bible Commentary, vol. 1 (Zondervan, 1995), p. 100; Gerhard Kittel and
Gerhard Friedrich, Theological Dictionary of the New Testament: Abridged in One Volume, ed. Geoffry Bromiley (Eerdmans 1985, reprinted 1992), s.v. basileia.
- ↑ Gospel and Kingdom: A Christian Interpretation of the Old Testament, p. 53.
- ↑ The Bible and the Future, p. 45.
- ↑ What is the Kingdom of God?
- What is the Kingdom of God?, by George Eldon Ladd
- The Kingdom of God and the Old Testament, by Graeme Goldsworthy
- The Kingdom of God, by Peter J. Leithart
- The Kingdom of God in Paul's Gospel (PDF), by Brian Vickers (SBJT)
- Common errors in understanding the Kingdom, by D. A. Carson
- What Was the Message of Jesus?, by Mark D. Roberts
- The Kingdom in Matthew, by Matthew Allen J.D.
- The Kingdom of God in New Testament Theology: The Battle, The Christ, The Spirit-Bearer, and Returning Son of Man, by Darrell L. Bock