The 12 disciples preached "the gospel of Jesus Christ" according to PROPHECY.
Whereas, apostle Paul preached "the gospel of Christ" according to the MYSTERY.
Mark wrote according to PROPHECY, Mr 1:1-2 KJV: "The beginning of the gospel of Jesus Christ, the Son of God; AS IT IS WRITTEN IN THE PROPHETS,"
Paul wrote according to the MYSTERY, Ro 16:25 KJV: "Now to him that is of power to stablish you according to my gospel,
and the preaching of Jesus Christ, ACCORDING TO THE REVELATION OF THE MYSTERY, which was kept secret since the world began,"
We KNOW the gospel Mark preached in NOT the same gospel Paul preached because we can READ Mark 1:1-2
and we know that Paul received FURTHER revelation from the ASCENDED Lord Jesus Christ that was kept secret until God later FIRST revealed it to Paul.
And, if you want to deny that, YOU ARE NOT A KJV BIBLE BELIEVER. PERIOD.
The gospel of Christ that saves today, the gospel of Christ that Paul preached, CANNOT BE FOUND OUTSIDE OF ROMANS THRU PHILEMON, KJV.
The "gospel of Christ" according to the MYSTERY, is a distinct gospel that Jesus, from heaven, FIRST gave exclusively to apostle Paul.
Paul's gospel of Christ is a unique gospel in that works of any kind is completely EXCLUDED.
Paul's gospel of Christ states we are saved by God's GRACE and mercy through faith.
Paul's gospel of Christ is BELIEVE plus nothing.
Paul's gospel of Christ also reveals the forgiveness of all sins the moment a person believes 1Cor 15:1-4,
how that Christ DIED FOR OUR SINS, was buried, and rose again the third day.
You simply won't find all those aspects of the gospel of Christ anywhere outside of Paul's writings, which are contained in Romans thru Philemon, KJV.
The "gospel of Jesus Christ" according to PROPHECY included repent and be water baptized.
Whereas, Paul's "gospel of Christ" DID NOT include water baptism, nor does Paul ever say that we must repent to be saved.
On the contrary, Paul emphasizes that we must simply BELIEVE the gospel without works for salvation by Grace.
And, he even warned us that if we add the works of the Law to his grace "gospel of Christ" that faith is made void,
the cross and the promise made of none effect, the reward is NOT reckoned of grace but of debt.
Paul says that if it be of works it is NO MORE GRACE.
Paul wrote, 1 Co 1:17 KJV: "For Christ sent me NOT to baptize, but to preach the gospel: not with wisdom of words,
lest the cross of Christ should be MADE OF NONE EFFECT."
Ro 4:14 KJV: "For if they which are of the law be heirs, FAITH IS MADE VOID, and the PROMISE MADE OF NONE EFFECT:"
Ro 4:4 KJV: "Now to him that worketh is the reward NOT RECKONED OF GRACE, but of debt."
Ro 4:5 KJV: "But to him that WORKETH NOT, BUT BELIEVETH ON HIM that justifieth the ungodly, HIS FAITH IS COUNTED FOR RIGHTEOUSNESS."
Eph 2:8-9 KJV: "For BY GRACE ARE YE SAVED through faith; and that NOT OF YOURSELVES: it is the GIFT of God:  NOT OF WORKS, lest any man should boast."
Ro 11:6 KJV: "And if by grace, then is it no more of works: otherwise grace is no more grace. But IF IT BE OF WORKS, THEN IS IT NO MORE GRACE:"
Ro 10:4 KJV: "For Christ is the end of the law for righteousness TO EVERY ONE THAT BELIEVETH."
Eph 1:13 KJV: "In whom ye also TRUSTED, after that ye HEARD the word of truth,
THE GOSPEL OF YOUR SALVATION: in whom also after that YE BELIEVED, YE WERE SEALED with that holy Spirit of promise,"
Don't deny what God has clearly SEPARATED in His word of truth.