This Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men.  Acts 2:23

In Acts 2:22-36, Peter is preaching a sermon to make the case that Jesus is the Christ—that He is God’s Messiah.  In his sermon he anticipates the objection that says, “If Jesus is the Messiah,  then why did He die like he did?”  Peter anticipating this objection says in Acts 2:23 that Jesus being arrested and tried by the Jews and crucified and killed at the hands of the Romans, was according to the definite plan and foreknowledge of God. 

Peter is making the point that God was very much in control and at work while Jesus was being crucified.  Many of the Jews had missed this in the Scriptures.  They had missed the fact that their prophets had clearly spoke of a messiah to come who would suffer and be crushed for sin.  That is why Peter and others spend a lot of time early on in ministry highlighting these truths to show that the suffering and death of Christ was all a part of God’s divine plan. 

We see this in Psalm 22, Isaiah 53 and elsewhere.  Isaiah said, 

He (Jesus) was pierced for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his wounds we are healed.  All we like sheep have gone astray; we have turned—every one—to his own way; and the LORD has laid on him the iniquity of us all.

Isaiah and others taught of a Messiah to come who was going to suffer and be killed—they taught that God’s messiah would come and be crushed for the sins of mankind. 

The Jews missed this.  So when Jesus died, many of them said “See, this man can't be the Messiah because he died."  But Peter and others used passages like the one in Isaiah 53 and in Psalm 22  to say “See, this man has to be the Messiah, look at the way in which He died.”  Jesus' suffering and death for sin was all a part of God’s plan and proves that He was God’s man.