What event does Peter refer to in Acts 15:7?

In Acts 15:7, Peter says, > “Men and brethren, you know that a good while ago God made a choice > among us, that the Gentiles by my mouth should hear the word of the > gospel and believe.” (KJV) What specific moment in time is Peter referring to here? Is he talking about his vision on the rooftop in Joppa (Acts 10:11), the actual baptism of Cornelius in Caesarea (Acts 10:48), or perhaps even earlier—back when Jesus was still with the apostles and said He would give Peter the keys of the kingdom of heaven (Matthew 16:19)?