Paul's Gospel versus the Gospel of Jesus
Paul in First Corinthians chapter 15 explains that his gospel is the acceptance as true certain facts that he was taught by the 12 apostles. Paul understood simply that acceptance or belief in these facts was the good news and if you believe them you were saved unless this news was untrue in the first place.
Here is what Paul says in First Corinthians 15:
15 Let me now remind you, dear brothers and sisters,[a] of the Good News I preached to you before. You welcomed it then, and you still stand firm in it. 2 It is this Good News that saves you if you continue to believe the message I told you—unless, of course, you believed something that was never true in the first place.[b]
3 I passed on to you what was most important and what had also been passed on to me. Christ died for our sins, just as the Scriptures said.4 He was buried, and he was raised from the dead on the third day, just as the Scriptures said. 5 He was seen by Peter[c] and then by the Twelve. 6 After that, he was seen by more than 500 of his followers[d]at one time, most of whom are still alive, though some have died.7 Then he was seen by James and later by all the apostles. 8 Last of all, as though I had been born at the wrong time, I also saw him. 9 For I am the least of all the apostles. In fact, I’m not even worthy to be called an apostle after the way I persecuted God’s church.
10 But whatever I am now, it is all because God poured out his special favor on me—and not without results. For I have worked harder than any of the other apostles; yet it was not I but God who was working through me by his grace. 11 So it makes no difference whether I preach or they preach, for we all preach the same message you have already believed. 1 Cor. 15:1-10 NLT.
This is a creedal Christianity -- it is what you believe that saves you. Here, Paul says holding to two beliefs saves you:  Jesus died for your sins and  Jesus resurrected. Nothing more. Paul repeats the latter in Romans 10:9 (NKJV) -"if you ...believe in your heart that God has raised him from the dead, you shall be saved." (For further discussion of Romans 10:9, see below.)
Paul ends by boasting that he did more than the 12 apostles to propagate this gospel, claiming God showed him more "favor" (grace) than the 12 in order to do so. This explanation does not expand the creeds we must have beyond the two mentioned. Thus, Paul clearly made it all about just two beliefs you need to continue to believe to be saved.
May I suggest the reason the 12 did less to promote this gospel of belief alone than Paul did was because it was only Paul's view of the gospel. Just reading the New Testament, we see Paul's view of the gospel in First Corinthians did not match the Gospel the true Jesus gave the 12 in the first place. Nor was it the Gospel Peter preached in Acts 3 nor what Apostle Philip preached in Acts 8, as I will show. Hence, indeed Paul worked harder than the 12 to promote this creedal Christianity but the reason is that the 12 were busy preaching the Gospel of the Kingdom that Jesus taught - a far different gospel than what Paul taught.
Jesus' Different Gospel
Before Paul, or the atonement or the resurrection, Jesus' was already preaching the Gospel: "I came to preach the Gospel of the Kingdom, that is the reason why I was commissioned." (Luke 4:43) Jesus' preaching never turned upon believing the two facts Paul claimed believing them would impart salvation. Instead, Jesus' gospel was already saving people like Zaccheus and the thief on the cross without mention of these truths even in a future sense.
For example, in the case of Zaccheus and the thief on the cross, it was under the moral suasion of Jesus' words or example that each repented from their prior behavior.
Zaccheus was a thief by excessive tax collecting, and he promised Jesus to pay four-fold back to the poor. This is a work-worthy-of-repentance prescribed in the Mosaic Law to return stolen money when you can no longer find the true owner. (Exodus 22:1-9; Numbers 5:5-8.) In response, Jesus said "salvation had come to this house" that day. (Luke 19:9.)
As to the thief on the cross, time was too short to do likewise, but he did do a work worthy of repentance too. For the thief had been mocking Jesus previously along with the other thief on the cross. Now the thief decided to reprove his fellow thief, and defend Jesus' innocence to the other thief. He did so at the likely risk of a speer spike in his side if the soldiers overheard him sympathizing with a convicted "rebel" like Jesus. The thief then acknowledged Jesus as his ruler -- "please remember me when you come into your kingdom." (Luke 23:42.) Jesus told him "this day you will be with me in paradise." Jesus' obedience unto death was a sermon all on its own to this repentant thief. It resulted in a work worthy of repentance - an open confession of Jesus as king of the Kingdom of God -- a synonym for the word Messiah. This fit Jesus' Gospel which always expressly or impliedly included "works worthy / appropriate of repentance" as a necessary step and means of salvation. See Works Worthy of Repentance.
Incidentally, you do not save yourself in Jesus' Gospel. Rather, your obedience to Christ's gospel saves you by God's favor / grace. For Christ told you to appease the one you have offended - God or man, and only after doing so can you come back to the altar and offer your "atonement" / "gift" (Matt 5:23-24).
Now we know Christ gave us that sacrifice on the cross as a means to offer it as an atonement to God through Christ's work. Christ's atonement saves you but only because you obeyed his instruction to do first a work worthy of repentance to appease the one you offended. Atonement operates whether you believe in Christ's atonement or not. So long as you satisfy the precedent works of repentance / efforts to appease the one you angered by sin. Thus, the obedience to Christ's commandment to do works worthy of repentance triggers the atonement's application to you, whether you fully know about the atonement or not.
However, Paul does not even mention accepting Jesus as Messiah, and thus never implies you must obey Jesus' commandments as your king to appropriate atonement. Paul only speaks elsewhere of the necessary act of confessing the "master / Lord Jesus" (Romans 10:8-9). Paul does not explain immediately what this means. Then two sentences later Paul explains he means simply "calling on the name" of the "Lord" "shall" save you. (Romans 10: 13.) This would indicate just saying "Lord Jesus save me" is all you need to do to appropriate salvation, as long as you believe in his atonement and resurrection too. Paul likely contemplated this confession was at Baptsm. For in Paul's trial testimony, he explained that at his own baptism, he was told to "call on the name of the Lord." Acts 22:16. This description does not necessarily involve confessing Jesus as Messiah, but only requires praying to Jesus by name to wash you. You are simply baptized into the "name of Jesus."
The consequence is that Paul never expressly tells us to accept Jesus as our King - Messiah as a vital step in salvation. He only tells us to call on the "name" of the Lord to save us at some undefined point, besides having the two beliefs identified in 1 Corinthians 15.
Thus, to be saved, Paul says in 1 Corinthians 15 and Romans 10:8-13, you need only believe in the atonement and the resurrection, and then ask "Master / Lord Jesus, save me" or "wash me" at some unspecified time, probably baptism.
These two passages - 1 Corinthians 15 and Romans 10:8-13 -- thus represent a total shift away from the Gospel of Jesus. Instead, you see the germ of a creedal Christianity. You can see the origin of the Nicene Creed of 325AD. The only major truths that were added in 325 AD at Nicea to these two passages were that there was now a necessary belief in a bi-deity of God the son ("true God from true God") and God the Father. (FYI: The Holy Spirit was not recognized until 381 AD as a separate person who also was God in a holy trinity.) See Constantine's Damage to Christianity. See Study Notes below on "Creedal Christianity."
The Obvious Invalidity of Paul's gospel
The facts which Paul recites as true in First Corinthians 15 and Romans 10 are of course true. Christ did die for our sins. Christ did resurrect. But people were saved by Jesus' Gospel before these events and afterwards. Never does Jesus ever say that believing either event saves you or is necessary to believe to be saved.
But the fact that such truths are true does not make them facts you simply believe and are saved. That is the heresy of Gnosticism... the knowledge (or gnosis in Greek) of a fact supposedly saves you. Even if atonement made Jesus your savior, and is the precondition of the Gospel of the kingdom with Jesus as Messiah prince, does not make the belief about the atonement and / or the resurrection the Gospel itself.
Watching an Airplane versus Getting on Board.
Let me illustrate. Assume God said salvation from prior and ongoing sin was, hypothetically, first appropriately repenting of earlier sin before going on board a plane and then submitting to the authority of the pilot for an entire flight to Jerusalem for a big party. If you rebelled during the flight, you must then do your first works of repentance. What are those? In this hypothetical, the works which caused God to respond with grace originally and thus previously issued you a ticket to get on board without paying the full fare, having explained that Jesus paid that full fare. If you do not repent during the flight after an act of rebellion, God tells you such sons of the kingdom are cast outside the plane in outer darkness just prior to landing in the city of Jerusalem as an invitee who was improperly dressed for the party.
Two facts must be true to make salvation possible in this scenario: the plane  can take off in the air (cf. resurrection of Jesus) and [b] it has enough fuel to get to Jerusalem (cf the adequacy of Christ's atonement). If either fact were untrue, you cannot fulfill the conditions of salvation as God in this hypothetical defined it.
However, it would be a categorical error to define salvation in this example as only having to believe the plane can take off and it has enough fuel. You cannot sit at the terminal, and waive goodbye to the occupants peering out from the plane's small windows, and think you are saved. You have to get on board. You have to accept the Pilot's rules and obey him for the duration of the flight to Jerusalem, including making amends when you rebel en route, ... rather than waiting at the terminal until nightfall (outer darkness) to hear of a safe landing in Jerusalem. You missed the opportunity to enter the kingdom party in Jerusalem by smugly thinking just believing in facts that make the trip possible is all you needed to do to somehow claim a right to be teleported to the Jerusalem party without ever getting on board as instructed.
Thus, Paul is incorrect that continuing to hold onto two beliefs was the gospel all by itself, sealed by simply saying "Master / Lord Jesus save me."
How can we know that for sure?
Two means: 1. examine what was the Gospel Jesus taught before the atonement and resurrection?; and 2. examine what was the post-atonement and post-resurrection Gospel the 12 understood, e.g., Apostle Peter's preaching in Acts 3 and Apostle Philip's preaching in Acts 8.
1. The Gospel of Jesus.
As discussed above, Jesus said he was already preaching the gospel prior to these events happening -- the atonement and resurrection. People were already getting saved under the gospel of the kingdom of God. Jesus preached that gospel that he declared saved Zaccheus and the thief without Jesus ever mentioning they must believe he would die for their sins and resurrect.
Jesus never taught belief in facts about his future atonement for mankind, or in his future resurrection, were necessary to believe to be saved. They were going to become true, but they were no more necessary to believe to become saved than was circumcision for a Gentile. The only facts Jesus ever said you had to believe to be part of his church were that (a) he was Messiah / prince / ruler and (b) he was the Son of God. (Matt 16:15-16.)
2. The Post-Atonement & Post-Resurrection Gospel.
Peter preached a sermon that led thousands to become saved. He told the crowd in Acts "repent and turn around, that your sins may be blotted out." (Acts 3:19.) Peter said that then the presence of God will send "refreshing" so that God may send "the Lord Jesus who was preached to you before whom heaven must receive until the times of restoration" (Acts 3:19-20). That is, Jesus is in heaven until the kingdom of God is established on earth. Then Peter draws them to follow the commands of Jesus whom was already "preached to you before."
Peter then quotes Deuteronomy 18:18-19. By quoting Deut 18:18-19, Peter was saying Jesus is "the Prophet" whom God told Moses He would send and whom God will hold all men accountable to have followed the words the Prophet speaks. Peter quotes the passage, saying "you shall hear all things whatever he speaks to you." (Acts 3:22). And again Peter quotes it, telling the crowd: "It shall be that every soul that will not hear that Prophet shall be utterly destroyed from among his people." (Acts 3:23.)
Peter in context is telling them to repent from their sin, and accept all commands from Jesus as from the Prophet -- a figure equated to Messiah in many commentaries and Rabbinical writings prior to Christ. See Shalom.org's The Messiah -The Prophet. Peter then says God sent and "raised up Jesus to bless you, in turning every one of you away from your iniquities." (Acts 3:26.)
Peter thus was all about Jesus as Messiah, and the necessity of repentance from sin toward obedience. Nothing is mentioned that you must believe Jesus atoned or resurrected. The resurrection is mentioned as a blessing on them to turn them from sin. A sign to touch their conscience. However, Peter did not say it was a belief, which if held, along with atonement, would save you by the mere believing these facts are true.
Philip likewise preached the Gospel of the Kingdom of God in Acts 8. Luke first records that Philip "preached Christ" (Messiah) unto a city of Samaria. (Acts 8:5.) The people "gave heed" to Philip's preaching. Then in verse 12 it says the people
"believed Philip's preaching concerning the Kingdom of God and the name of Jesus Christ [and] they were baptized, both men and women." (Acts 8:12.)
Nothing here is mentioned about Jesus' atonement or resurrection.
So preaching "Christ" meant he taught Jesus was Messiah. What did it mean Philip preached the "name of Jesus Christ and baptism"? Well, all baptisms in the book of Acts were solely into the name of Jesus (not any other names.) This is reflected in Paul's own account in court of what Ananias said to Paul after his Damascus experience: "And now why are you waiting? Arise and be baptized, washing away your sins, calling on the name of the Lord." (Acts 22:16.)
Hence, likely Philip preached that baptism would wash away their sins, just as Peter did. Yet, nothing here is spoken of supposedly two essential beliefs about the atonement and resurrection that if you held such beliefs that holding them continuously saves you, unlike what Paul says in 1 Corinthians 15.
There is thus a complete disconnect between Paul's gospel which relies exclusively in belief in facts ABOUT Jesus, versus a Gospel Jesus taught about submitting to him as Messiah / King and as The Peophet of Deuteronomy 18 and doing works worthy of repentance for salvation, including baptism. Jesus is a WAY of salvation that is "agonizing" to stay on, and not fall away. It requires severe repentance -- going to heaven maimed or you can go to hell whole. (Mark 9:42-47.)
Paul's way by contrast is the easy cost-free way of belief alone -- a wide road that is easy to find and broad enough that you need never worry about falling away from such a broad road.
The Pauline Gospel is so at odds with Jesus' Gospel that Paulinism teaches deliberately that we can ignore Jesus' words, casting them off to a prior defunct dispensation.
But the truth is the opposite: it is the truths from Jesus that would set you free if you obeyed them as if he were your King / Messiah and the Prophet of Deuteronomy 18. Salvation is not about needing to believe in unquestionable truths about the atonement and resurrection. That's why the early church called its message The WAY, not The CREED THAT SAVES. For a thorough review of Jesus' gospel, please read our book free online - Jesus Words on Salvation.
Creedal Christianity. When Emperor Constantine as Pontifex Maximus -- head of all religions at Rome -- orders the church to meet at 325 AD in Nicea, and drafts the Nicene creed, Constantine has appropriated a Pauline view of Christianity. He prefers a religion that is all about a creed or set of beliefs. Constantine did so because in his primary faith in the Sun God this is all you need -- there are no behavioral standards other than to worship the god of the Sun known as Sol Invictus.
This means the Nicene Creed was the revival of Pauline Christianity. Paul's brand of Christianity had existed in a few small areas until it died off under the influence of apostle John (see link) and the battering critique about Paul by the church writer Tertullian in 207 AD in his work entitled Against Marcion. See link.
Constantine revived this Paul-two-beliefs-gospel so he could co-opt such a creedal Christianity as the best means to insert a new version of Christ. In the Nicene Creed, Jesus was not only son of God and Messiah, but also was now God the son. This was to accomplish Constantine's decree of 321 AD that everyone must rest on Sunday to honor the venerable -- worship-worthy -- Sun. For Constantine was a sun worshiper and his god was god the son and god the Sun known as SOL INVICTUS which means the invincible sun.
In this religion of Sol Invictus, the sun in the sky is god the Son, and he was born every December 25 (the winter solstice) by the mating of god the father Horus with his god-wife Osiris. So the way Constantine obviously figured he could bring Christianity into his personal pagan belief was to call this god the Sun by the name of Jesus. In this way, Constantine could win over Christians to change the worship day from Sabbath to Sunday, as Constantine decreed 4 years earlier -- in 321 AD -- expressly to honor the "worship-worthy Sun." Not Jesus!
From 325 AD onward, Christianity within the Roman sphere of influence now was made to worship and rest only on Sunday. But outside the Roman sphere of influence, such as in the middle east and in the Eastern Orthodox Church in particular, the day of rest and worship then and now has always been the Sabbath day … what we know today as Saturn's day / Saturday. See The Sabbath Command.