Acts 28:23 And when they had appointed him a day, there came many to him into his lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and out of the prophets, from morning till evening.

Paul met the Lord Jesus on the road to Damascus and believed on Him. First thing, he went to the Jews in the synagogues and preached Jesus as the Christ, the Son of God. Paul preached Jesus as the Son of God and proved He is Christ. Paul was preaching from the law and the prophets. Paul was preaching Jesus as Christ and the Son of God as the foundation of his gospel; the gospel he received from revelations from the Lord Jesus Christ.

Jesus himself and the twelve apostles preached, as the foundation, Jesus is Christ, the Son of God. They preached this foundation of Christ and fully revealed the gospel of the Kingdom; that Christ would establish His kingdom on the earth.

After the resurrection Peter preached the gospel of the circumcision. Jesus resurrection ensured his return to establish the Kingdom.

Paul preached the same foundation of Jesus as Christ, the Son of God, but revealed his gospel of faith alone in the death and resurrection of Christ for justification to eternal life.