According to The Wycliffe Bible Commentary, the kingdom of heaven “refers to the Messianic kingdom promised in the OT, of which Jesus was about to be presented as king.”  Wycliffe goes on to note that the term kingdom of God “often has a wider connotation, but usually in the Gospels the two are used interchangeably.”

Additional perspectiveis provided by Unger’s Bible Dictionary, which states,

The “kingdom of God” is evidently a more comprehensive term than the “kingdom of heaven” and embraces all created intelligences both in heaven and on earth who are willingly subject to God and thus in fellowship with him.  The “kingdom of heaven,” more precisely the “kingdom of the heavens,” is a term descriptive of any type of rulership God may assert on the earth at a given period  . . . .

Matthew Henry’s Commentary on the Whole Bible adds that the kingdom of heaven has been opened “to all believers, by the death and resurrection of Jesus Christ.  It is a kingdom of which Christ is the Sovereign. . . . It is a kingdom of heaven, not of this world, a spiritual kingdom. . . .”

But, is the kingdom of heaven (or the kingdom of God) a specific place or location?  If so, why did Jesus Christ say in Luke 17:21, “[T]he kingdom of God is within you”?  [Note:  When we quote Scripture in this article, we use the wording in the New King James Version of the Bible.]

Before addressing the question as whether or not the kingdom of heaven is a specific place or location, it is important to understand that the term “heaven” is not synonymous with the term “kingdom of heaven.”  The Bible is clear that heaven is a place (e.g., see John 14:1-3).  Unger’s notes that there are actually three heavens and that the third heaven “is the abode of the Triune God. Its location is unrevealed.”

Wycliffe explains that the Greek word translated as “within in Luke 17:21 may mean “among,” a viewpoint that Matthew Henry acknowledges.  Wycliffe goes on to say, “Jesus asserted that the kingdom of God was already present and needed only to be recognized.  He had brought the kingdom with him and was living among them.”  This explanation is compatible with the view expressed earlier by Unger’s and it seems to suggest that the kingdom of heaven began while Jesus Christ was still ministering on Earth.

However, if the kingdom of heaven (or the kingdom of God) began during Christ’s ministry, why did Jesus say, “the kingdom of heaven [or, the kingdom of God] is at hand” (Matthew 4:17; 10:7; Mark 1:15) or that some of those to whom He was speaking would “not taste death till they see the kingdom of God . . .” (Mark 9:1; Luke 9:27)?  These statements indicate that the kingdom of heaven had not yet come.

The key to answering the first part of the question seems to be clarification of the Greek words that have been interpreted “is at hand.”  There are two points of view in this regard.

According to Wycliffe, the Greek words interpreted as “is at hand” can also mean “has come near,” an interpretation that is supported by Strong’s Concordance of the Bible.  Thus, it can be argued that the Greek words do not allude to a future time, but to the physical proximity of the Messiah, Jesus Christ.  If this is correct, then those to whom Jesus was speaking did not have to wait for the kingdom of heaven to come; it was essentially right there in the midst of them, if they trusted in Christ for their eternal salvation.

On the other hand, if Jesus intended to indicate that there would be a relatively short waiting period before the kingdom of heaven would come, He may have been referring to the subsequent coming of the Holy Spirit, which occurred on the day of Pentecost, 50 days after Christ’s death on the cross.

And, why would Jesus Christ say that some of those to whom He was speaking would “not taste death till they see the kingdom of God”?  The logical answer is that He would not have made such a statement, if the kingdom of heaven (or the kingdom of God) began during His first advent (i.e., during His ministry on Earth).  Therefore, it seems reasonable to conclude that the kingdom of heaven did not begin during the ministry of Jesus, but instead it began shortly later, after Jesus ascended to heaven and the Holy Spirit made His Own presence known on Earth.

However, after Jesus describes in Luke 21:7-28 the events that will occur before His second advent (i.e., His second coming), He concludes in verse 31 that “when you see these things happening, know that the kingdom of God is near.”  This suggests that the kingdom of God (or the kingdom of heaven) will not come until after Christ makes His triumphant return to Earth.  [For a discussion of Christ’s return to Earth, click on “Discrepancies as to When Jesus Christ Will Return]

Conclusion

So, what should we conclude as to when the kingdom of heaven will come?  For all the pieces of the puzzle to fit, there seems to be only one plausible explanation:  The kingdom of heaven began when the Holy Spirit made His presence known on the day of Pentecost, but the kingdom of heaven will not be enjoyed to its ultimate extent until after Jesus Christ makes His triumphant return to Earth.

Thus, the believers (i.e., Christians) who are living on Earth (i.e., those who have not yet experienced physical death) can enjoy benefits of being in the kingdom of heaven (happiness, peace, joy, etc.), but not to the extent that they will be able to enjoy them after Christ returns and begins his reign on Earth.

This concept is similar to what the Bible teaches regarding the eternal salvation of Christians.  The benefits of a person’s eternal salvation begin when he (or she) trusts in Jesus Christ as Savior, but the culmination of that person’s eternal salvation will occur when he (or she) goes to be with Christ in heaven.