The expression “kingdom of God” occurs some 68 times in the New Testament, while the phrase “kingdom of heaven” occurs only 32 times. Importantly, while “kingdom of God” is found in ten New Testament books, all the references to the “kingdom of heaven” occur in the Gospel of Matthew.
While the “two kingdoms” advocates suggest that Matthew was describing a “millennial kingdom” and the other Gospel writers were speaking of a “universal kingdom,” there is no biblical basis to presume this. On the contrary, there is good biblical indication that the two expressions are not referring to two different things, but are two ways of referencing the same thing.
We see this in the words of Christ himself in his conversation with his disciples after he spoke with the rich young ruler who did not want to give up his possessions to follow Christ:
“Truly I tell you, it is hard for someone who is rich to enter the kingdom of heaven. Again I tell you, it is easier for a camel to go through the eye of a needle than for someone who is rich to enter the kingdom of God” (Matthew 19:23-24, emphases added).
Looking closely at these verses we see that Christ used both expressions – the kingdom of God and the kingdom of heaven – at the same time and clearly speaking about the same thing.
Why then does Matthew usually use only the expression “kingdom of heaven”? The answer is simply because Matthew originally wrote his Gospel to a Jewish audience that by tradition – in order to not break the commandment against lightly using God’s name – often tried to avoid speaking the name of God and frequently substituted a euphemism. As a result, the Jews frequently referred to the “kingdom of heaven” rather than the “kingdom of God” – using the one expression for the other in much the same way that we nowadays might say “The White House announced” meaning the President or his administration announced something.
The other Gospels usually used “kingdom of God” where Matthew used “kingdom of heaven” in recording their parallel accounts of the same spoken words. We can see this by comparing, for example, Matthew 11:11-12 with Luke 7:28; or Matthew 13:11 with Mark 4:11 and Luke 8:10. In these and many other instances Matthew used the expression “kingdom of heaven” in deference to his Jewish audience, while the other Gospels used “kingdom of God.”
For all these reasons, the New Testament makes it clear that the kingdom of heaven and the kingdom of God are exactly the same thing.