God had stood by Paul and had helped him, as He had promised ( Acts 26:22; cf Acts 26:17). Paul preached a message thoroughly in harmony with Israel"s faith (cf. Acts 3:18; Acts 17:3). Acts 26:23 may be Luke"s condensation of Paul"s exposition of many Old Testament messianic prophecies that Jesus fulfilled (e.g, Isaiah 42:6; Isaiah 49:6; Isaiah 53:10; Isaiah 60:3). Many of the Jews rejected the ideas of a suffering Messiah, His resurrection from the dead, and direct ministry to Gentiles, but Paul found support for these in the Old Testament. [source][source][source]
"Here in substance is the Gospel that Paul preached and that believers ought always to proclaim, "that Christ died for our sins according to the scriptures; and that he was buried, and that he rose again the third day according to the scriptures" ( 1 Corinthians 15:3-4)." [1][source]