What was the Gospel of the Kingdom that people had to believe up through Acts 8 and Acts 9?
The Gospel of the Kingdom was the message that people had to believe Jesus is the Christ, the Son of the living God. It did not yet include the revelation of salvation by grace through His death, burial, and resurrection. This belief in His identity persisted through Acts 8 and even into Acts 9.
Les explains that the confession Peter made at Caesarea Philippi—"Thou art the Christ, the Son of the living God"—was the required belief all the way from Christ's earthly ministry through Acts chapter 8 and even into chapter 9. He notes that God had not yet revealed the tremendous plan of salvation based upon grace, that Christ died for the sins of the world, was buried, and rose from the dead. Up till then, people were to believe who He was. That is the Gospel of the Kingdom. The account of Philip baptizing the Ethiopian eunuch in Acts 8 illustrates this, with no argument or room for dispute.