The Central Truth of Paul’s Message Was the Resurrection

Paul was imprisoned for the hope and resurrection of the dead (Acts 23: 6).

At Paul’s examination before Felix Paul declared that he had “hope toward God…that there shall be a resurrection of the dead, both of the just and the unjust” (Acts 24: 15).

And now I stand and am judged for the hope of the promise made of God unto our fathers: (7) Unto which promise our twelve tribes, instantly serving God day and night, hope to come. For which hope's sake, king Agrippa, I am accused of the Jews.
Why should it be thought a thing incredible with you, that God should raise the dead? (Acts 26:6-8).

Paul met with the Jews at Rome and informed them that it was “for the hope of Israel” that he was bound with a chain (Acts 28: 20).

The central truth of Paul’s message was the resurrection, not salvation by grace. This does not mean that Paul did not teach salvation by grace.  It just wasn’t the driving force behind him. The gospel message had at its core the lordship of King Jesus and the resurrection of Jesus. The death of Jesus was in-order-to the resurrection. Further confirmation of this is found in Romans 10:9 and Philippians 2:8-11.

Romans 10:9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.

Philippians 2:8-11 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. (9) Wherefore God also hath highly exalted him, and given him a name which is above every name: (10) That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; (11) And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.

The Commissioned Gospel Message

The lordship of the historical person of Jesus Christ
The resurrection by God of this Jesus from the dead
God commands everyone to repent of his sins
Jesus is ordained of God to be the judge of the quick and the dead
Through the name of Jesus is the forgiveness of sins