You will find certain people saying that there is a difference between the ‘kingdom of heaven’ and the ‘kingdom of God’; but my difficulty is to know what the difference is. Why does Matthew talk about the kingdom of heaven rather than the kingdom of God? Surely the answer is that he was writing primarily for the Jews, and to the Jews, and his chief object, perhaps, was to correct the Jewish conception of the kingdom of God or the kingdom of heaven. They had got into this materialistic way of looking at the kingdom; they were thinking of it politically and in a military sense, and our Lord’s whole object here is to show that His kingdom is primarily a spiritual one. In other words He says to them, ‘You must not think of this kingdom primarily as anything earthly. It is a kingdom in the heavens, which is certainly going to affect the earth in many different ways, but it is essentially spiritual. It belongs to the heavenly rather than to the earthly and human sphere.’
What is this kingdom, then? It means, in its essence, Christ’s rule or the sphere and realm in which He is reigning. It can be considered in three ways as follows. Many times when He was here in the days of His flesh our Lord said that the kingdom of heaven was already present. Wherever He was present and exercising authority, the kingdom of heaven was there. You remember how on one occasion, when they charged Him with casting out devils by the power of Beelzebub, He showed them the utter folly of that, and then went on to say, ‘If I cast out devils by the Spirit of God, then the kingdom of God is come unto you’ (Matt 12:28). Here is the kingdom of God. His authority, His reign was actually in practice. Then there is His phrase when He said to the Pharisees, ‘the kingdom of God is within you, or, ‘the kingdom of God is among you’ (NAS “is in your midst” Luke 17:21). It was as though He were saying, ‘It is being manifested in your midst. Don’t say “look here” or “look there”. Get rid of this materialistic view. I am here amongst you; I am doing things. It is here.’ Wherever the reign of Christ is being manifested, the kingdom of God is there. And when He sent out His disciples to preach, He told them to tell the cities which received them not, ‘Be ye sure of this, that the kingdom of God is come nigh unto you.’ (Luke 10:9, 11, cf Luke 19:11, 21:31)
It means that; but it also means that the kingdom of God is present at this moment in all who are true believers…In writing to the Colossians he gives thanks to the Father ‘who hath delivered us from the power of darkness, and hath translated us into the kingdom of his dear Son’ (see note Colossians 1:13). The ‘kingdom of his dear Son’ is ‘the kingdom of God, it is ‘the kingdom of heaven’, it is this new kingdom into which we have entered. Or, again, in his letter to the Philippians he says, ‘Our conversation is in heaven,’ or, `Our citizenship is in heaven.’ We are here on earth, we obey the powers that be, we live our lives in this way. Yes; but ‘our citizenship is in heaven; from whence also we wait for a Saviour’ (see note Philippians 3:20). We who recognize Christ as our Lord, and in whose lives He is reigning and ruling at this moment, are in the kingdom of heaven and the kingdom of heaven is in us. We have been translated into the ‘kingdom of his dear Son’; we have become a ‘kingdom of priests. (cf 1Pe 2:9, 10 1Pe 2:9; 10, Rev 1:6, Rev 5:10)
The third and last way of looking at the kingdom is this. There is a sense in which it is yet to come. It has come; it is coming; it is to come. It was here when He was exercising authority; it is here in us now; and yet it is to come. It will come when this rule and reign of Christ will be established over the whole world even in a physical and material sense. The day is coming when the kingdoms of this world will have become ‘the kingdoms of our Lord, and of his Christ, when Jesus shall reign where’er the sun Doth his successive journeys run; His kingdom stretch from shore to shore, Till moons shall wax and wane no more.
It will then have come, completely and entirely, and everything will be under His dominion and sway. Evil and Satan will be entirely removed; there will be `new heavens and a new earth, wherein dwelleth righteousness’ (2Pe 3:13), and then the kingdom of heaven will have come in that material way. The spiritual and the material will become one in a sense, and all things will be subject to His sway, that ‘at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; and that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father’ (Php 2:10, 11). (Lloyd-Jones, D. M. Studies in the Sermon on the Mount)