The scene illustrates a passage from the Acts of the Apostles (7:54-60). Stephen was one of the seven deacons chosen by the Apostles to assist them; accused by the Jews, he was ordered to be stoned at the city gates, and as this was happening, his face shone, and praying, he asked God not to impute the sin to his executioners.
When the members of the Sanhedrin heard this, they were furious and gnashed their teeth at him. But Stephen, full of the Holy Spirit, looked up to heaven and saw the glory of God, and Jesus standing at the right hand of God. “Look,” he said, “I see heaven open and the Son of Man standing at the right hand of God.” At this they covered their ears and, yelling at the top of their voices, they all rushed at him, dragged him out of the city and began to stone him. Meanwhile, the witnesses laid their coats at the feet of a young man named Saul. While they were stoning him, Stephen prayed, “Lord Jesus, receive my spirit.” Then he fell on his knees and cried out, “Lord, do not hold this sin against them.” When he had said this, he fell asleep. (Acts 7:54-60, NIV)