What did Peter remember at the Jerusalem council that helped Paul's defense?
Peter remembered his earlier experience at the house of Cornelius, where God had opened his eyes to the truth that Gentiles could be saved by simply believing, without first becoming proselytes of Judaism. That divine precedent helped Paul's defense at the Jerusalem council, showing God Himself had already accepted believing Gentiles.
Les explains that Peter came to Paul's defense at Jerusalem and remembered how God had sent him to the house of Cornelius a good while earlier. There Peter's eyes were opened to the fact that God could save Gentiles without their becoming proselytes of Judaism. In Acts 15:12 the multitude then kept silence and listened to Barnabas and Paul declare what miracles and wonders God had wrought among the Gentiles. Les emphasizes that had Peter not had that eye-opening experience, Paul would never have gotten off the hook, and Christianity would never have gotten off the ground. God sovereignly opened Peter's eyes twelve years earlier to the fact that Gentiles could be saved by simply believing.