by No one is saying that there is more than one gospel, or what is to be believed for God to declare one righteous, it is the fact that there is only one gospel in a particular age that is to be believed.
What Jesus preached in His earthly ministry was prophecy, Acts 3:21, that He was the one sent by God to be Israel’s Messiah, and that the Kingdom, that was first prophecied by Daniel, 2:44, was at hand.
By believing this, Israel would be saved (declared righteous).
This was Peter’s confession, Matt. 16:16,17.
We know on this side of the cross that Israel (nationally) did not accept that, Jn.1:11, Rom.11:11.
What Peter and the eleven preached at Pentecost, and after, was still that message, and when they would preach the death, burial and resurrection, it was in context of history and not FOR salvation. In fact it was accusatory for what they (Israel) did, Acts2:23,36; 3:14,15; 5:30. And they needed to repent of that and accept the Jesus of Nazareth was/is THEIR Messiah, Acts.2:37,38; 3:19,20.
In 1Cor.15:11, in context Paul is in the middle of discussing the resurrection, and when he mentions the “other apostles” it is in the context that there IS a resurrection, for all who knew/knows the scriptures understood that there IS a resurrection, but some at that time were questioning that fact.
Paul’s gospel, given to him by the Resurrected and ascended Lord Jesus Christ, was a secret hid in God Himself, Rom.16:25; Eph.3:9. It’s purpose is to gather an heavenly people unto Himself to reign with Christ in the heaven, Phil.3:20.
This gospel/message of Christ dying for the sins of man, being buried, and rising again the third day, 1Cor.15:1-4, was given to ALL, and not just to Israel. This was/is done in spite of Israel, that salvation coming to the world, instead of through her blessing, which was prophecied about in Isaiah.
The book of Acts shows this transition from the gospel of the Kingdom, Matt. 3:1,2, 4:17, 10:7, to the gospel of the grace of God/Christ, Acts 20:24; Rom.1:16.
Satan thought he won at the cross, by Israel being pushed by Satan to crucify Jesus, he thought that God couldn’t fulfill His plan for the earth through Israel.
Satan made the error that would bring his absolute doom, while he was focused on the earth, God revealed His secret to Paul, and that is to reclaim and redeem the heaven first, with an entity never prophecied about called the body of Christ, with Christ being the Head of the body, Eph.1:22,23.
If Satan would have had any idea about this he would not have had the Lord crucified:
1Cor.2:7 “But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: 8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.”
The “mystery,” is just that, and it is not to be joined to the gospel of the Kingdom which is prophecy. God will redeem the earth through Israel, and will redeem the heaven through the body of Christ.
Therefore, Christ is Glorified in Heaven and in Earth….
Paul says that there is no other foundation than Jesus Christ…TRUTH.
That being said, he also said that every man should take heed on how he builds on that foundation:
For the earth: Thou art the Christ, the Son of the living God, the King of Israel.
For the heaven: Christ died for my sins, and was buried and was raised again the third day, the Head of the body.
Of course His shed blood covers all of those that were saved before the cross, and makes what God told them in time past for to be declared righteous, legal.
It will do the same in the future tribulation and 1000 year reign, when the gospel of the Kingdom is once again proclaimed, and then of course the “everlasting gospel”…