Ephesians 6:19 And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,
The mystery of the gospel was revealed first to Paul.
Romans 16:25 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,
16:26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith:
Paul and the twelve Apostles preached Christ. Paul preached Christ according to the mystery and the scriptures of the prophets. The twelve did not preach Christ according to the mystery.
Paul claims the gospel as being "my gospel", which distinguishes it from all others. Paul states his gospel was according to the mystery. The mystery was kept secret until revealed to Paul, which means the twelve Apostles could not have known it.
Paul's gospel is now made manifest.
Ephesians 3:1 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
Paul was an Apostle to the Gentiles.
3:2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
God clearly states that there are dispensations. We are in the dispensation of the grace of God. Gentiles can now be saved freely, along with Jews, without the law and covenants. Gentiles no longer are seperated from Israel. Gentiles no longer have to come to God through Israel.
3:3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
Jesus revealed the mystery to Paul.
3:4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
3:6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
The church, the body of Christ was revealed first to Paul.
Colossians 1:25 Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;
1:26 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
The mystery was hid even from the twelve disciples, but was made manifest to us by Paul.
Luke 18:31 Then he took unto him the twelve, and said unto them, Behold, we go up to Jerusalem, and all things that are written by the prophets concerning the Son of man shall be accomplished.
18:32 For he shall be delivered unto the Gentiles, and shall be mocked, and spitefully entreated, and spitted on:
18:33 And they shall scourge him, and put him to death: and the third day he shall rise again.
18:34 And they understood none of these things: and this saying was hid from them, neither knew they the things which were spoken.
Paul's gospel includes the death, burial, and the resurrection of Christ (1 Corinthians 15:1-4). The twelve Apostles had no understanding of any of this according to Luke 18:34, even when Jesus told them of his death and resurrection in Luke 18:33 and in fact, it was hid from them. The twelve Apostles could not have been preaching the same gospel as Paul.
Matthew 4:23 And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people.
Mark 1:14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God,
1:15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
Jesus was preaching the gospel of the promised Davidic kingdom (Luke 1:32).
Luke 9:1 Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases.
9:2 And he sent them to preach the kingdom of God, and to heal the sick.
9:6 And they departed, and went through the towns, preaching the gospel, and healing every where.
The twelve Apostles were not preaching the same gospel as Paul, but rather the gospel of the kingdom, which Paul never preached.
It is imperative to believe the gospel that was given to us and be justified unto eternal life.
The gospel has been progressively revealed throughout scripture. Those that believed the gospel revealed in their dispensations, believed God, and were justified unto eternal life (Romans 4:3). If you only believe that the kingdom is at hand (Mark 1:15), as preached by the twelve, you are lost. If you only believe in the name of Jesus, as preached by the twelve, that he is the Christ, the Son of God (John 20:31), you are lost. If you only believe you should repent and be baptized (Acts 2:38), as preached by the twelve, you are lost. If you only believe the gospels of the twelve Apostles you are lost.
The gospel given us in this dispensation is only found in the epistles of Paul; examples being Romans 3:19-26 and 1 Corinthians 15:1-4.