The core of the message of Jesus is, "Repent, for the kingdom of God is at hand." And it was and still is.
The core message of the Gospel of the Kingdom is "Repent, for the Kingdom of God is at hand."
Doesn't mean we equate that to the Gospel of Jesus Christ.
Christ did preach the Gospel, so to speak, when He revealed it to His Disciples, but, He was not going out and preaching it to the world. In fact, He did not even send His Disciples to the world until after His death, after they were Baptized with the Holy Ghost:
Matthew 10
King James Version (KJV)
5 These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not:
6 But go rather to the lost sheep of the house of Israel.
7 And as ye go, preach, saying, The kingdom of heaven is at hand.
Acts 1:8
King James Version (KJV)
8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
The Gospel of the Kingdom was relevant to Israel in that it was to her the promises were relevant:
Romans 9
King James Version (KJV)
3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh:
4 Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises;
5 Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen.
It was to them that the promise of the Kingdom they expected to arise was given:
Luke 19:10-11
King James Version (KJV)
10 For the Son of man is come to seek and to save that which was lost.
11 And as they heard these things, he added and spake a parable, because he was nigh to Jerusalem, and because they thought that the kingdom of God should immediately appear.
The Kingdom they looked for, which was based on the Prophecy of the Hebrew Scriptures, was not a Kingdom in which Messiah could die. That is why Peter rebukes the Lord in opposition to the Gospel, and denies he even knows Him when He is taken. That too was prophesied:
John 16:32
King James Version (KJV)
32 Behold, the hour cometh, yea, is now come, that ye shall be scattered, every man to his own, and shall leave me alone: and yet I am not alone, because the Father is with me.
Continued...