Note: All Scripture quotations found in this article are from the text of the Authorized King James Version of the Bible.
The Kingdom of God and Kingdom of Heaven are two different kingdoms in scripture. Simply, the word "God" does not mean "Heaven". Making the two the same will cause problems and contradictions in the Bible.
In Matthew 5, the Kingdom of Heaven is spoken about and it says that if one sinned against God that he would go to hell. While Paul taught that the Kingdom of God is where the Christian lived(Romans 14:17). If the two are same then there is a contradiction that the Christian's salvation includes works.
Matthew 25:14-30 talks about the Kingdom of Heaven where an unfaithful servant goes to hell. Luke 19:12-27 talks about the Kingdom of God where the unfaithful servant loses his rewards and this matches up with the Christian at the Judgment Seat of Christ(2 Cor. 5:10). There is obviously a difference between the two kingdoms.
According to Matthew 25:1, the parable of the ten virgins is connected to the Kingdom of Heaven. When the bridegroom comes, the foolish virgins are shunned at the wedding because they did not keep their lamps burning. The bridegroom professes that he does not know them(Matthew 25:1-10). This is not the Christian's Kingdom of God because Christians are know by Christ(John 10:14,27) and are not virgins(plural), rather they are the Bride of Christ(Ephesians 5:22-32, Revelation 19:7-9). The Church is likened unto chaste virgin(singular) in 2 Corinthians 11:2, but does not match the plural virgins in Matthew 25 that follow the Bride as her companions in the wedding(Psalms 45:14).
The Kingdom of God is a spiritual kingdom. In Matthew 6:25-33, it talks about not worrying about physical things, but rather spiritual things, which is the Kingdom of God and his righteousness.
Romans 14:17 "For the kingdom of God is not meat and drink; but righteousness, and peace, and joy in the Holy Ghost."
The Kingdom of God is not a physical place to live in, but rather is within the believer when he is Born Again.
Luke 17:20 "And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation: 21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you."
John 3:3 "Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God."
The Kingdom of Heaven is a physical kingdom that will take place on earth and consists of violence. Remember that the earth is in the heaven(Genesis 1:14-16).
Matthew 11:12 "And from the days of John the Baptist until now the kingdom of heaven suffereth violence, and the violent take it by force."
The first place where the Kingdom of Heaven is mentioned is where John the Baptist was preaching about the the fulfillment of Isaiah 40. John was preaching about the Kingdom of Jesus Christ on this earth. Other Old Testament prophecies also reveal that the Kingdom of Heaven was physical. Read Genesis 49:10; Isaiah 2:1-5, 49:5-13; Psalms 2; Jeremiah 23:5 and Luke 1:30-33.
Matthew 3:2 "And saying, Repent ye: for the kingdom of heaven is at hand. 3 For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight."
Jesus tells the Jews that the Kingdom of Heaven is at hand in Matthew 4:17. This proves that it is physical because it shows a kingdom coming on earth and is not referring to someone dying. The book of Matthew mentions the Kingdom of Heaven several times speaking of an earthly rule of Israel that is talked about in Old Testament prophecies(Matthew 5:1-6, 13, Matthew 19:10-30).
Matthew 4:17 "From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand."
The gospel that Jesus Christ preached for Israel to the Jews was the Kingdom of Heaven(Matthew 4:23, 5:4, 8:11-12, 19:28, 20:16, 25:31-34). The Jews knew that this gospel that Jesus was preaching was the physical reign on earth(John 1:41,45,49, 6:14-15, 12:13). The disciples of Jesus Christ knew that the Kingdom of Heaven was on earth like King David and Solomon's kingdom. Notice in the below verse, Jesus does not rebuke them and this proves that it is physical.
Acts 1:6 "When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel? 7 And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power."
The physical kingdom was appointed to Adam in Genesis 1:28. Adam was also in dominion over a spiritual kingdom also until he fell(Genesis 1:27, Genesis 5:1-5). When Adam fell he lost the image of God and the spiritual kingdom was gone.
1 Corinthians 15:45 "And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit. 46 Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual. 47 The first man is of the earth, earthy: the second man is the Lord from heaven. 48 As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly. 49 And as we have borne the image of the earthy, we shall also bear the image of the heavenly. 50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption."
After Adam, we see God setting up His physical kingdom on earth through the Kings of Israel. The Devil also appoints physical kingdoms on earth(Deut. 13:12-16, Luke 4:6, 2 Cor. 4:4). The Devil rules the kingdom of Assyria and Egypt through the king of Assyria and Pharaoh in Ezekiel 31 and 29. Jeremiah recognized that God's physical kingdom ended at the Babylonian Captivity(Lamentations 5:16).
Satan is referred to the god of this world in 2 Corinthians 4:4 and is in charge of worldly kingdoms that he can give to people(Luke 4:6). In Daniel 10:13, the Devil was the ruler of the kingdom of Persia during the times of the Gentiles.
When the New Testament begins Jesus comes into the world and the Jews remember that God promised them that he would restore His physical kingdom as talked about earlier in the article. Jesus preached about both the Kingdom of Heaven and the Kingdom of God(John 4:24). He preached about both because he can rule both at the same time, which explains why he talks about them with conjunction with each other sometimes in the Gospels. The Kingdom of Heaven was put off for Israel because they rejected Jesus as King and chose a Gentile king instead.
John 19:15 "But they cried out, Away with him, away with him, crucify him. Pilate saith unto them, Shall I crucify your King? The chief priests answered, We have no king but Caesar."
Jesus Christ prophesied that the Kingdom of God would be inside his saints in Luke 17:20-21. Later on, Paul told Christians that they were now working in the Kingdom of God(Romans 14:17-18, Colossians 1:13, 4:11). The spiritual kingdom that Adam lost was now back for the Christians after being Born Again.
Luke 17:20 "And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation: 21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you."
During the Millennium at the Second Coming of Jesus Christ, both the Kingdom of God and Heaven will be on the earth. Jesus will reign physically on this earth and also spiritually.
Revelation 11:15 "And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever."
Revelation 20:4 "And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years."
So remember, the two kingdoms are different. The Kingdom of God is spiritual and the Kingdom of Heaven is physical. Remembering these two facts will help you understand more things in scripture once you start looking for the occurrences of these phrases. Failing to realize the difference can lead Bible readers in doctrinal errors, such as Christians losing their salvation, working for their salvation and even going through the Tribulation.
