In Judaism at that time, it would be very hard to put 2 things together: 1, that God forgives sins and 2, that this forgiveness would come through the ordeal of a person who claimed to be the Messiah, when Messiah they thought would be a victory securing figure for independence. The Gospel was always that God forgives sins (see David in Ps 32). It was a huge barrier, however, to realize that this Jesus of Nazareth was going to provide forgiveness through his ordeal, because it wasn't supposed to happen. This is the dilemma the apostles find themselves in in the 'blind spot' verses of Lk 18 etc.
So you do not know what was being preached in that gospel? And all this time I thought that you were the resident expert!
Frankly, it is not difficult to understand what was being preached:
"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 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 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).
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).
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).
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.
