What did Peter realize in Acts 15 about his earlier ministry to Cornelius?
Peter realized that years earlier in Caesarea God had used him to bring the Word to Cornelius, a Gentile, and that he had witnessed that Gentile's salvation. This showed God's long-term purpose to open the Word to Gentiles, not to require circumcision or Law-keeping.
Les explains that the Judaizers and Jewish leaders, including Peter and the eleven, were coming down hard on Paul for teaching grace to Gentiles, trying to convince him that Gentile believers had to practice circumcision and keep the Law. Paul adamantly refused. Then in Acts 15 Peter wakes up, and Les says the Holy Spirit gave him a big push. Peter realizes that years ago in Caesarea God had used him to take the Word to a Gentile, Cornelius, and Peter witnessed that Gentile's salvation. God had the long term in view, not the near, and had Peter primed and ready. Though the text does not say twelve years, chronologically that is what it was. Peter says a long time ago, or a good while ago, God by his mouth opened the Word to the Gentiles. In verse 10 he asks why test God by putting a yoke on the neck of the disciples, meaning the believers in Antioch who had come to Paul's Gospel, not those under Law or legalism, which the Judaizers were trying to impose.