'Rightly Dividing the Word'
Jesus Preached Kingdom Gospel
Luke 4:43-44 but He said to them, "I must preach the kingdom of God to the other cities also, because for this purpose I have been sent." And He was preaching in the synagogues of Galilee.
Jesus believed God sent John the Baptist to preach repentance and the gospel of the kingdom to save the Jews from damnation when the Old Covenant had finally ended.
Since He was also sent by God to preach the gospel of the kingdom, He humbled Himself to John the Baptist as the last OT Prophet, by preaching John's kingdom gospel.
Kingdom of Heaven Fulfilled
Matthew 10:5-7 These twelve Jesus sent out and commanded them, saying: "Do not go into the way of the Gentiles, and do not enter a city of the Samaritans. But go rather to the lost sheep of the house of Israel. And as you go, preach, saying, 'The kingdom of heaven is at hand.'
Since the kingdom of heaven was near with the coming judgement of God upon the Jews under the Mosaic law, so out of their love John, Jesus and the Apostles were fervently preaching the gospel of the kingdom.
The very day that Jesus was crucified, it was the great and dreadful day of the Lord for the Jews, when the Old Covenant was obsoleted or terminated by God, thus ending the Mosaic law.
After the resurrection of Jesus, the saints of the Old Covenant rose from the graves and they inherited the kingdom of heaven thus fulfilling the prophecy that 'the kingdom of heaven is near'
With the termination of Judaism, Jews were no longer the people of God. Being unsaved, and like the gentiles, they needed Jesus for salvation. So Jesus warned them that unless they repent of their wicked and evil ways, all of them would be cursed for only devout Jews (those who believed Moses), would believe and accept Him as their Messiah.
John 8:24 "Therefore I said to you that you will die in your sins; for if you do not believe that I am He, you will die in your sins."
How the kingdom of God was fulfilled?
Luke 17:20-21 Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, "The kingdom of God does not come with observation; nor will they say, 'See here!' or 'See there!' For indeed, the kingdom of God is within you."
Our Lord Jesus reiterated that He would suffered and be killed, and on the third day He would rise from the dead and this is the gospel of grace in the New Covenant of His blood
Immediately after His resurrection, He appeared to all His disciples. Thomas was the last of His Apostles to believed His resurrection. After all His Apostles had believed and accepted the gospel of grace, Jesus gave each of them the Holy Spirit of redemption, thereby starting the Body of Christ.
With God, who is the King of kings, establishing His kingdom in each Apostle's physical body as His temple, this fulfilled the prophecy of the kingdom of God is at hand
John 20:21-22 So Jesus said to them again, "Peace to you! As the Father has sent Me, I also send you." And when He had said this, He breathed on them, and said to them, "Receive the Holy Spirit."
Mark 4:11 And He said to them, "To you it has been given to know the mystery of the kingdom of God; but to those who are outside, all things come in parables,
The death of Jesus ended the Old or Mosaic Covenant, meanwhile His death also started the New Covenant of His blood.
The resurrection of Jesus fulfilled the kingdom of heaven when the saints of the OT were taken back to heaven. At the same time it fulfilled the kingdom of God with the King of kings establishing His kingdom within the physical body of each Apostle.
