The kingdom of heaven is that kingdom on God’s earth in which heaven is interested, which heaven promotes, with Christ as its King – that spiritual kingdom on earth which prepares for heaven citizens who discern Christ as the Lord from heaven – that kingdom which acts as the portal for and leads to heaven.
The Lord Jesus Christ introduced many new concepts to the attention and understanding of the New Testament Church and this is one of them. Only the Gospel of Matthew uses this term. The kingdom draws attention to the fact that the King had arrived among them. “of heaven’” distinguishes it from “the kingdom of God” which does not come with observation Lk 17:20. “of heaven” reminds us that it is not an earthly kingdom Jn 18:36 but it is a kingdom which reaches to heaven, which brings the King’s subjects to heaven, and in which heaven is interested.
This kingdom is not of this world; and one can be in the kingdom of heaven but not of the kingdom of heaven.
It is Christ’s kingdom which He has yet to deliver spotless and faultless to His Father on the Day of Judgment Mat 13:41,43; 1Cor 15:24; Heb 2:13.
Theologically, the kingdom of heaven is known as the visible Church, to distinguish it from the invisible Church which is known in Scripture as the kingdom of God. The similarity in terms is because of the similarity in concept and identity; the difference in terminology teaches and highlights that most fundamental of doctrines – “you must be born again” Jn 3:3,5.