The Gospel of Paul which he calls my gospel is the death and resurrection of Jesus Christ. This is the content of the gospel which saves and in which the church stands. Was this the gospel which Jesus and his disciples taught?
1 Cor. 15:3-5
3 For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He rose again the third day according to the Scriptures, 5 and that He was seen by Cephas, then by the twelve.
Of course when Jesus was alive on earth his death had not yet happened. The gospel being preached was called the gospel of the kingdom of God. Was there a previous mention of the Kingdom of God in reference to Jesus?
Mark 1:14-15 (NKJV)
14 Now after John was put in prison, Jesus came to Galilee, preaching the gospel of the kingdom of God, 15 and saying, “The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel.”
Zacharias who had been visited by the angel Gabriel and being released from his curse of being rendered mute, announced the Kingdom.
67 Now his father Zacharias was filled with the Holy Spirit, and prophesied, saying:
68 “Blessed is the Lord God of Israel,
For He has visited and redeemed His people,
69 And has raised up a horn of salvation for us
In the house of His servant David,
70 As He spoke by the mouth of His holy prophets,
Who have been since the world began,
71 That we should be saved from our enemies
And from the hand of all who hate us,
72 To perform the mercy promised to our fathers
And to remember His holy covenant,
73 The oath which He swore to our father Abraham:
74 To grant us that we,
Being delivered from the hand of our enemies,
Might serve Him without fear,
The Kingdom will redeem His people who is Israel. He will provide for salvation for the House of David. The kingdom, Israel, will be saved from their enemies who are the surrounding nations. The holy covenant promised to Israel which is the promise of the land. The goal is to serve before God in holiness and righteousness in the days of their life on earth.
8 Also I give to you and your descendants after you the land in which you are a stranger, all the land of Canaan, as an everlasting possession; and I will be their God.”
9 And God said to Abraham: “As for you, you shall keep My covenant, you and your descendants after you throughout their generations. 10 This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised;
The holy covenant is the promise of the land. This promise is bilateral, the people of Israel must practice circumcision. This is the kingdom gospel. It is a promise to Israel. What does Paul think about the content of the gospel of the ministry of Jesus?
Romans 15:8 (NKJV)
8 Now I say that Jesus Christ has become a servant to the circumcision for the truth of God, to confirm the promises made to the fathers,
Jesus preached circumcision and confirmed the promises made to the fathers. Paul did not preach circumcision. Paul did not confirm the promises made to fathers, he preached to the Gentiles.
8 To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, 9 and to make all see what is the fellowship[a] of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ.
Paul preached to a new audience to the Gentiles, not to Israel alone. Paul preached the mystery and not the promises made to the fathers. You cannot find his gospel of salvation by faith alone, the death and resurrection of Jesus and the saints, in the Old Testament.
Did Jesus and the disciples preach the death and resurrection of Jesus Christ in order to be saved? The disciples were preaching the gospel, they were promised to sit on twelve thrones judging the people of Israel. Were they saved? The gospel preached by the disciples did not believe in the death and resurrection of Jesus Christ until these events actually happened.
31 Then He took the twelve aside and said to them, “Behold, we are going up to Jerusalem, and all things that are written by the prophets concerning the Son of Man will be accomplished. 32 For He will be delivered to the Gentiles and will be mocked and insulted and spit upon. 33 They will scourge Him and kill Him. And the third day He will rise again.”34 But they understood none of these things; this saying was hidden from them, and they did not know the things which were spoken.
Did Peter, the one called the rock who was appointed by Jesus to be the leader of the church after his death did he believe in the resurrection of Jesus Christ? Not only did Peter not believe but he was so deluded by Satan that Jesus rebukes him.
Mark 8:30-33 (NKJV)
29 He said to them, “But who do you say that I am?” Peter answered and said to Him, “You are the Christ.” 30 Then He strictly warned them that they should tell no one about Him.
30 Then He strictly warned them that they should tell no one about Him. 31 And He began to teach them that the Son of Man must suffer many things, and be rejected by the elders and chief priests and scribes, and be killed, and after three days rise again. 32 He spoke this word openly. Then Peter took Him aside and began to rebuke Him. 33 But when He had turned around and looked at His disciples, He rebuked Peter, saying, “Get behind Me, Satan! For you are not mindful of the things of God, but the things of men.”
Before the death of Jesus, He and his disciples preached a gospel of the kingdom. This was not the gospel Paul preached. Their gospel preached circumcision and the establishment of the promised Kingdom to Israel. The content of the gospel changed from the Kingdom gospel to the gospel preached by Paul.