2:34 “For David did not ascend into the heavens, but he says himself:
‘The Lord said to my Lord,
“Sit at My right hand, 35 till I make Your enemies Your footstool.”’
36 “Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.”
37 Now when they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?”
Three times every year the Israelites were to come to the Temple which at this time was in Jerusalem and had been for centuries. The new year began for them with the celebration of Passover, celebrating the last plague in Egypt when the angel of death passed over every home that was covered by the blood of the Lamb. That same night they left Egypt and bondage never to return. As a memorial of God's great deliverance He instituted an 8 day feast. Seven weeks later they were to return to the Temple for Pentecost or the feast of first fruits (shavuot). This feast was to remind them that all they had was a gift from God. The third time they came was in the fall of the year for the feast of tabernacles or feast of booths. At that time everyone lived in temporary shelters for a week commemorating the 40 year journey in the wilderness to remind them always that God kept His promise and brought them into the Promised Land.
Why the review? To make one simple point. Jesus was crucified at Passover when the city was full of worshippers from all over the world. These same worshippers have now returned for Pentecost. The men and women listening to Peter were part of the mob screaming "Crucify Him! Crucify Him!!!"
Now for the very first time they realize Who He was and what they participated in. Peter lead by the Spirit of God shows them from their own sacred writings the undeniable proof. King David was not writing about himself for he is dead and buried and everyone knows it. The only One who died yet never saw corruption is Jesus. He alone fulfills all that the prophets spoke and wrote about.
I can imagine that for a time you could hear a pin drop as the crowd processed in stunned silence the implications of what they have just seen and heard. Their Messiah was here and they killed Him... The text says they were "cut to the heart". It is a expression used a few times in Scripture. It means they knew without a shadow of a doubt that what they were hearing was the truth.
When they recovered enough to speak their reaction was humble and direct. "Brethren what do should we do?"
It's too late to fix anything so now what? We'll talk about that tomorrow.