Why did God have Peter remember the Cornelius event at the Jerusalem council?
God had the long-term view, not just the near, so He prompted Peter to recall Cornelius. That memory confirmed God had already opened the Word to Gentiles and saved a Gentile through Peter, proving Gentile believers were not required to be circumcised or keep the Law.
Les explains that the Jerusalem council was the setting where Judaizers and Jewish leaders, including Peter and the eleven, were pressing Paul to require circumcision and Law-keeping for Gentile believers, but Paul refused. Then in Acts 15 Peter "wakes up," and the Holy Spirit gives him a push to remember Caesarea. God had used Peter to take the Word to Cornelius, and Peter had witnessed that Gentile's salvation. Because God had the long-term view, He had Peter primed and ready. Peter testified that a good while ago God by his mouth opened the Word to the Gentiles. He then asked why they should test God by putting a yoke on the disciples—the Antioch believers saved by Paul's Gospel, not under Law or legalism. So the memory confirmed Gentile salvation apart from circumcision and Law.