Who did Saul realize Jesus of Nazareth really was?
Saul came to recognize that Jesus of Nazareth, whom he had hated and tried to stamp out, was the God he thought he was worshiping. He had to bring the two together, realizing the one he persecuted was the true God of Abraham, not an imposter or blasphemer.
Les explains that Saul of Tarsus had to recognize that the one he was hating and trying to stamp out—this Jesus of Nazareth—was the God he thought he was worshiping. Saul had thought Jesus was an imposter, a blasphemer, and a fake, and that the best thing he could do as a servant of the true God of Abraham was to stamp out anybody connected to Jesus. In Acts 26:9-10, Paul admits he verily thought he ought to do many things contrary to the name of Jesus of Nazareth, shutting up saints in prison and putting them to death for believing Jesus was the Christ. Saul was the chief advocate of this persecution until he had to recognize the truth.