The following is an excerpt from The MacArthur New Testament Commentary on Matthew 3.

Now in those days John the Baptist came, preaching in the wilderness of Judea, saying, “Repent, for the kingdom of heaven is at hand.” (Matthew 3:1)

Although the precise phrase is not found there, the kingdom of heaven is basically an Old Testament concept. David declares that “the Lord is King forever and ever” (Ps. 10:16; cf. 29:10), that His kingdom is everlasting, and that His dominion “endures throughout all generations” (Ps. 145:13). Daniel speaks of “the God of heaven [who] will set up a kingdom which will never be destroyed” (Dan. 2.:44; cf. Ezek. 37:25), a “kingdom [that] is an everlasting kingdom” (Dan. 4:3). The God of heaven is the King of heaven, and the heavenly kingdom is God’s kingdom.

Matthew uses the phrase kingdom of heaven thirty-two times, and is the only gospel writer who uses it at all. The other three use “the kingdom of God.” It is probable that Matthew used kingdom of heaven because it was more understandable to his primarily Jewish readers. Jews would not speak God’s name (Yahweh, or Jehovah), and would often substitute heaven when referring to Him-much as we do in such expressions as “heaven smiled on me today.”

There is no significant difference between “the kingdom of God” and the kingdom of heaven. The one phrase emphasizes the sovereign Ruler of the kingdom and the other emphasizes the kingdom itself, but they are the same kingdom. Matthew 19:23–24 confirms the equality of the phrases by using them in interchangeably.

The kingdom has two aspects, the outer and the inner, both of which are spoken of in the gospels. Those aspects are evident as one moves through Matthew. In the broadest sense, the kingdom includes everyone who professes to acknowledge God. Jesus’ parable of the sower represents the kingdom as including both genuine and superficial believers (Matt. 13:3–23), and in His following parable (vv. 24–30) as including both wheat (true believers) and tares (false believers). That is the outer kingdom, the one we can see but cannot accurately evaluate ourselves, because we cannot know people’s hearts.

The other kingdom is the inner, the kingdom that includes only true believers, only those who, as John the Baptist proclaimed, repent and are converted. God rules over both aspects of the kingdom, and He will one day finally separate the superficial from the real. Meanwhile He allows the pretenders to identify themselves outwardly with His kingdom.

God’s kingly rule over the hearts of men and over the world may be thought of as having a number of phases. The first is the prophesied kingdom, such as that foretold by Daniel. The second phase is the present kingdom, the one that existed at the time of John the Baptist and that he mentions. It is the kingdom that both John and Jesus spoke of as being at hand (cf. 4:17). The third phase may be referred to as the interim kingdom, the kingdom that resulted because of Israel’s rejection of her King. The King returned to heaven and His kingdom on earth now exists only in a mystery form. Christ is Lord of the earth in the sense of His being its Creator and its ultimate Ruler; but He does not presently exercise His full divine will over the earth. He is, so to speak, in a voluntary exile in heaven until it is time for Him to return again. He reigns only in the hearts of those who know Him as Savior and Lord. For those “the kingdom of God is … righteousness and peace and joy in the Holy Spirit” (Rom. 14:17).

The fourth phase can be described as the manifest kingdom, in which Christ will rule, physically, directly, and fully on earth for a thousand years, the Millennium. In that kingdom He will rule both externally and internally-externally over all mankind, and internally in the hearts of those who belong to Him by faith. The fifth, and final, phase is the “eternal kingdom of our Lord and Savior Jesus Christ,” which “will be abundantly supplied” to all of His own (2 Pet. 1:11).

Had God’s people Israel accepted their King when He first came to them, there would be no interim kingdom. The kingdom at hand would have become the kingdom of a thousand years, which, in turn, would have ushered in the eternal kingdom. But because they killed the forerunner of the King and then the King Himself, the millennial kingdom, and consequently the eternal kingdom, were sovereignly postponed.