The Gospel of the kingdom was that the millennial kingdom was ah um, in their midst, offered by the King Himself. “and saying, "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."”
**Mark *1:15 *NASB
There is more to the "gospel of the kingdom" than that:
John the Baptist said that he came baptizing with water so that the Lord Jesus might be revealed to Israel:
"I myself did not know him, but the reason I came baptizing with water was that he might be revealed to Israel" (Jn.1:31).
After seeing the spirit descending on the Lord Jesus the Baptist said,
"and I saw, and bore witness that this is the Son of God" (Jn.1:34).
The announcement that the kingdom was near at hand would naturally lead the Jew to expect that the Christ would be revealed and this was the meaning of the Baptist's words here:
"I indeed baptize you with water unto repentance. but He that cometh after me is mightier than I, whose shoes I am not worthy to bear: He shall baptize you with the Holy Spirit, and with fire: Whose fan is in his hand, and He will throughly purge His floor, and gather His wheat into the garner; but He will burn up the chaff with unquenchable fire" (Mt.3:11-12).
The Lord Jesus Himself made it plain that salvation among the Jews depended on their recognizing His "identity":
"And he said unto them, Ye are from beneath; I am from above: ye are of this world; I am not of this world. I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am He, ye shall die in your sins" (Jn. 8:23-24).
We can know that the Lord Jesus was preaching that He is the Christ and the "Son of God" by the following conversation between the Lord and some Jews who did not believe in Him:
"Then came the Jews round about him, and said unto him, How long dost thou make us to doubt? If thou be the Christ, tell us plainly. Jesus answered them, I told you, and ye believed not: the works that I do in my Father's name, they bear witness of me" (Jn.10:24-25).
Later in the same exchange we see that the same Jews wanted to stone Him because He said that He is the Son of God:
"Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, I am the Son of God?" (Jn.10:36).
So included in the "gospel of the kingdom" was the preaching that Jesus is the Christ, the Son of God. And that is exactly what Paul preached to the Jews:
"And straightway he preached Christ in the synagogues, that he is the Son of God...proving that this is the very Christ" (Acts 9:20,22).
Then after the rejection of that kingdom “...Then it goes and takes along with it seven other spirits more wicked than itself, and they go in and live there; and the last state of that man becomes worse than the first. That is the way it will also be with this evil generation."”
**Matthew *12:45 *NASB
The Jews would receive another chance to believe in Him after His crucifixion because when He was on the Cross the Lord Jesus prayed to the Father, saying
"Father, forgive them; for they know not what they do" (Lk. 23:34).
Then Peter told the nation if she repented then the Lord Jesus would be sent back so they could enjoy
"the times of refreshing" from His presence:
"Repent ye therefore, and be converted, that your sins may be blotted out, that the times of refreshing shall come from the presence of the Lord; And he shall send Jesus Christ, which before was preached unto you" (Acts 3:19-20).
These words of Peter were the offer of the kingdom.
Then “and that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning from Jerusalem. You are witnesses of these things.”
**Luke *24:47-48 *NASB
Beginning from Jerusalem my son
We have an uninterrupted sermon preached by Peter in Jerusalem on the day of Pentecost beginning at Acts 2:14 and ending at Acts 2:36. In that sermon there is not a word about God's "grace" and there is not a word about the purpose of the Lord Jesus' death on the Cross.
Instead, Peter used the facts concerning the Lord Jesus' death and resurrection in order to prove that He is the promised Messiah and then he summed up his discourse by saying:
"Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:36).
Again, the burden of this "good news" of Christ concerns the "identity" of the Lord Jesus.
Son, you must learn that the "good news" of Christ that the Lord Jesus died for our sins is not the same "good news" of Christ that Jesus is the Christ, the Son of God.