"And when the day of Pentecost was fully come, they were gathered with one accord in one place..."–Acts 2:1
The people are together in one place, on the right day, and of one accord, so Peter takes the opportunity to declare to them what the Scriptures say concerning sin, the Saviour, salvation, and the sentence against the ungodly. Then he gives to them a great warning, "Hear these words," which declare to us that Jesus is the Messiah, our only hope of salvation.
What a mercy it is to be within hearing of these words. Why these words? They are concerning Christ! He was "approved" of God. He was despised and condemned by men but not by God. The Lord God approved His person and work and gave Him power to work miracles. His death confirmed who He was. The Father "delivered Him up" to die. Through His death, divine justice would be satisfied, sinners would be saved, God and man would be brought together, and Christ would be glorified.
Then God "raised him up." The same one who delivered Him to death, delivers Him from death. He did not leave His soul in Hell. Christ was bound to die because of our sin debt, but when justice was satisfied, He could not remain there. His death was unto God a "sweet smelling savor". He saw the travail of His soul and was satisfied.
You will never see what the prophets spoke of, what the patriarch waited for, nor the end of the promises, until you see Christ. This is the "faithful saying worthy of all acceptation". God made Jesus both "Lord and Christ."
They were shocked when they heard this. You can know if this Gospel has touched your heart if you have cried out to God, "What shall we do?" Here was a people who had grown up trusting in the sufficiency of their religion to save them, they had seen Jesus crucified in weakness and disgrace, and had been told by their rulers that He was a fake. Peter told them that they had a hand in His death. Now, their first reaction was to get mad. They were cut to their hearts. They saw their wickedness. They did not go back to their religious leaders as they had done before. Now they cried "what shall we do?" They saw themselves undone.
Their case is sad but it is not hopeless. Peter says "repent and be baptized." Repent and admit you did not know it all. Admit you are hopeless. Admit you cannot save yourself. Submit to Christ. Bow to Christ. Until a work of grace has been done in you, you will neither confess your hopelessness nor submit to Christ. "Everyone of you," from the best to the worst. There is grace enough in Christ for all. Grace is suited to each one. This promise is open to all whom God calls.