From Acts of the Apostles, by William S. Kurz, SJ, commenting on Acts 4:8-12
Jesus instructed his disciples not to be afraid when they are brought to trial for their faith: “When they take you before synagogues and before rulers and authorities, do not worry about how or what your defense will be or about what you are to say. For the holy Spirit will teach you at that moment what you should say” (Luke 12:11–12). Here, in fulfillment of Jesus’ prophetic instructions, Peter is filled with the holy Spirit as he answered them.
His response manifests Spirit-inspired wisdom and boldness. The same Peter who cowered before the challenge of a serving girl (Luke 22:56–57) now fearlessly confronts the Sanhedrin: Leaders of the people and elders. He implicitly reproaches them for interrogating him about a good deed done to a cripple, then solemnly proclaims to them and to all the people of Israel the cause of the healing: it was done in the name of Jesus Christ the Nazarene. Even though Jesus was the one whom you, the Sanhedrin, crucified by handing him over to Pilate with a capital charge (Luke 23:1–2), God reversed that action and raised him from the dead. Peter declares that in his name, Jesus’ name, the lame man stands before you healed.
In this testimony Peter states the facts and, in so doing, reproves the Sanhedrin for opposing God’s plan. Jesus is the stone rejected by you, the builders which has become the cornerstone. The Sanhedrin are the “builders” of Israel, yet they had rejected the Messiah, the cornerstone, or capstone, essential to the building’s structure. In the Gospel, Jesus quoted this same line from Ps 118:22 to show that his rejection by the Jewish leaders was part of God’s plan and to illustrate the divine reversal of human purposes (Luke 20:17–19). God raised up the crucified Jesus and made him the cornerstone of the new temple, the Church.