Kingdom of Heaven / Kingdom of God

The "kingdom of Heaven" and the "kingdom of God" mean the same thing. Matthew is the only book in which the phrase "kingdom of Heaven" is used, and the only book in which both phrases are used. All other authors, including Paul, uses the phrase "kingdom of God". Both phrases are references to the kingdom that was to be established at Pentecost.

Daniel prophesied of the coming of this Kingdom (Daniel 2:44). It was established during the reign of the fourth kingdom of iron, which we know today was the Roman Empire. Also, as Daniel prophesied, it was broken into pieces a scattered to many nations, as is evident by Paul's travels and the churches he established, and consumed those nations. Today, it is still broken into pieces and consumes all nations, because every where that believers in Christ are present, so also is the kingdom of God.

John the Baptist preached of this kingdom, which at the time of his ministry, was only "at hand", or very close, or within reach, but not yet obtainable. The kingdom of Heaven would remain "at  hand" until Pentecost.

In Matthew 21:43, since God dwelled in the temple among the nation of Israel, Israel was a physical kingdom of God. However, later in Matthew 23:38 and Luke 13:35, Jesus declares that the temple is left desolate, signifying that God would no longer dwell in the temple made with hands. Under the New Testament, God would dwell among His people only in the form of the Holy Ghost in a spiritual kingdom. Passages such as John 14:2, 1 Corinthians 3:9, Ephesians 2:21-22, 1 Peter 2:5, describe bodies of believers as buildings, spiritual houses, and habitations of God. These are all alternate descriptions of the kingdom of God. They are realms in which God the Father and Christ are Kings and Lords over the groups of people.

The spiritual kingdom of God became available at Pentecost. From that time until 70 A.D., both the physical kingdom of God, the nation of Israel, and the spiritual kingdom of God coexisted. Mark 9:1 was fulfilled in 70 A.D. when the physical nation of Israel was completely destroyed by the Roman Legions which had surrounded it. (See the works of Flavius Josephus: War of the Jews - Books 5, 6, & 7.)

The kingdom of God is never used as a reference to immortal heaven, or rather the heaven where we will be when we depart this world. It is a spiritual place that exists here on earth. It is a place of refuge and of rest for the believers.

According to 1 Corinthians 6:9-10, only the righteous can inherit the kingdom of God. And according to 1 Corinthians 15:50, "flesh and blood cannot inherit the kingdom of God".  John 3:5 teaches that a person enters into the kingdom of God, but only after both regeneration and conversion. Since flesh and blood cannot inherit the kingdom of God, it can only be received by way of the Holy Spirit. This is also confirmed by John 3:5, since a person can only receive the earnest their inheritance, the Holy Spirit (Ephesians 1:14), by conversion, which also means to be born of the Spirit. Therefore, only the obedient children, those which have been exercised by the chastening of God and become fruitful (Hebrews 12:11), can possess the indwelling Holy Spirit and enter into the kingdom of God.

To enter into the kingdom of God is to also enter into Christs' rest. Only those who convert (Matthew 11:28-29) and believe (Hebrews 3:18, Hebrews 4:9-11) in Christ can enter into His rest.