The "gospel of the kingdom" was the announcement of the approach of the rule of heaven upon the earth. The Lord Jesus proclaimed:
"Jesus came into Galilee, preaching the gospel of the kingdom of God, And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel" (Mk.1:14-15).
John the Baptist also said that the kingdom is at hand:
"In those days came John the Baptist, preaching in the wilderness of Judaea, And saying, Repent ye: for the kingdom of heaven is at hand" (Mt.3:1-2).
He also 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).
The following conversation between the Lord Jesus and His disciples concerned His "identity":
"When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? And they said, Some say that thou art John the Baptist: some, Elias; and others, Jeremias, or one of the prophets. He saith unto them, But whom say ye that I am? And Simon Peter answered and said, Thou art the Christ, the Son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it" (Mt.16:13-18).
This confession of Peter, that the Lord Jesus is the Christ, the Son of God, is the "rock" upon which the Jewish church was to be built. Alfred Edersheim, a Jew who converted to Christianity, explains the meaning of the "rock" here:
"Perhaps it might be expressed in this somewhat clumsy paraphrase: 'Thou art Peter (Petros)--a Stone or Rock--and upon this Petra--the Rock, the Petrine--will I found My Church...so Christ promised that He would build His Church on the Petrine in Peter--on his faith and confession" (Edersheim, The Life and Times of Jesus the Messiah; Part Two [Grand Rapids: Wm. B. Eerdmans Publishing, 1971], 83-84).
At the time when the Lord Jesus walked the earth the "gospel of the kingdom" said nothing about His impending death. At Luke 9:2 we see that the Lord Jesus sent the Twelve Apostles to "preach the kingdom of God." Then later we can see that those Apostles did not even know that He was to die (Lk.18:31-34). Therefore, we can know that at this stage the "gospel of the kingdom" said nothing about the death and resurrection of the Lord Jesus.
We can also see that after the leaders of the nation of Israel plotted His death the Lord Jesus told His disciples not to make Him known:
"Then the Pharisees went out, and held a council against him, how they might destroy him. But when Jesus knew it, he withdrew himself from thence: and great multitudes followed him, and he healed them all; And charged them that they should not make him known" (Mt.12:14-16).
The kingdom of heaven, which the Lord Jesus had said was "at hand" at the beginning of His ministry, would now be delayed until He returned to earth after His death and resurrection and after He had ascended into heaven. The Lord Jesus said:
"And then shall they see the Son of man coming in a cloud with power and great glory. And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh. And he spake to them a parable; Behold the fig tree, and all the trees; When they now shoot forth, ye see and know of your own selves that summer is now nigh at hand. So likewise ye, when ye see these things come to pass, know ye that the kingdom of God is nigh at hand" (Lk.21:27-31).
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).
Later, on the day of Pentecost, Peter told these same Jews that they crucified Him "through ignorance." He also used the facts concerning His 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).
Later, after hearing the gospel preached the Ethiopian treasurer asked to be baptized with water: "And Philip said, If thou believeth with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God" (Acts 8:37).
When Paul was converted he was given a ministry to the Gentiles as well as the Jews (Acts 9:15). Here he mentions the ministry which he had among the Jews:
" And now, behold, I know that ye all, among whom I have gone preaching the kingdom of God, shall see my face no more" (Acts 20:25).
The first message which Paul preached after being converted was the "gospel of the kingdom" in the synagogues of 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).
That was the same gospel which Paul continued to preach to the Jews:
"And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures, Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ" (Acts17:2,30).
That is the same message that Apollos preached to the Jews:
"For he mightily convinced the Jews, and that publicly, shewing by the scriptures that Jesus is Christ" (Acts18:28).
So we can clearly see that the central message that was preached to the Jews is the fact that it is Jesus Who is the Christ, the Son of God.