A kingdom is not a simple matter. For example, the United States as a nation, a kingdom, is not a simple matter. The kingdom of God includes many things that need to be understood. In the Old Testament there is the kingdom of Israel. In the New Testament there is the kingdom of the heavens. Then after the church age there will be a period of a thousand years known as the millennium. The millennium is a kingdom of a thousand years (Rev. 20:4, 6). If we read the Scriptures carefully, we will see that even in the millennium there are some further divisions. The kingdom of Israel in the Old Testament, the kingdom of the heavens in the New Testament, and the millennial kingdom after the church age are parts of the kingdom of God. The kingdom of God runs from eternity to eternity. It is a realm, a sphere, in which God can rule and exercise His authority. From eternity past to eternity future there is such a thing called the kingdom of God. In this kingdom God exercises His authority to rule over all things.
Concerning the kingdom of Israel being a part of the kingdom of God in the Old Testament times, Matthew 21:43 says, “Therefore I say to you that the kingdom of God shall be taken from you and shall be given to a nation producing its fruit.” The kingdom of God was already there with the Israelites, but because they did not bring forth fruit, the Lord told them that the kingdom of God would be taken from them. That it could be taken from them means that the kingdom of God was among them already. The Lord took the kingdom from them and gave this kingdom to another people, which is the church.
We must also see from the Scriptures that the kingdom of the heavens is different from the kingdom of God. Most Christians think that the kingdom of the heavens and the kingdom of God are synonymous terms. When John the Baptist came, he told the people to repent, for the kingdom of the heavens had drawn near (Matt. 3:2). This meant that up until the time of John the Baptist, the kingdom of the heavens had not yet come but had only drawn near. Then when the Lord Jesus came out to preach the gospel at the beginning of His ministry, He said the same thing: “Repent, for the kingdom of the heavens has drawn near” (4:17). Later, in Matthew 10:7, the Lord sent the first group of disciples and told them to say the same thing, that the kingdom of the heavens had drawn near. When the Lord came out to preach the gospel, the kingdom of the heavens had not come yet. On the one hand, the kingdom of God was already among the children of Israel. On the other hand, the kingdom of the heavens was coming.
Matthew 12:28 says, “But if I, by the Spirit of God, cast out the demons, then the kingdom of God has come upon you.” In this verse the Lord refers to the kingdom of God, not the kingdom of the heavens. Even then, the kingdom of the heavens had still not come, but the kingdom of God was there by that time.
Another important passage showing this distinction between the kingdom of the heavens and the kingdom of God is Matthew 11:11-12: “Truly I say to you, Among those born of women there has not arisen one greater than John the Baptist, yet he who is least in the kingdom of the heavens is greater than he. But from the days of John the Baptist until now, the kingdom of the heavens is taken by violence, and violent men seize it.” This word indicates that up to this time “the kingdom of the heavens” had not come, and John the Baptist was not in it, because the smallest in the kingdom of the heavens is greater than John. Also, from the time of John until the time the Lord spoke this word was the period of time in which people could endeavor to get into the kingdom of the heavens.
In Matthew 16:19 the Lord told Peter that He would give to him “the keys of the kingdom of the heavens.” Even at the time of Matthew 16 the kingdom of the heavens had not begun, because the keys had not been given. The Lord gave the first key of the kingdom of the heavens to Peter on the day of Pentecost. The kingdom of the heavens started on the day of Pentecost when the church began to be built up. Peter used one key to open the gate for the Jewish believers to enter the kingdom of the heavens on the day of Pentecost (Acts 2:38-42), and he used the other to open the gate for the Gentile believers to enter the kingdom of the heavens in the house of Cornelius (10:34-48).