VERSES 51-53: Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye. (52) Which of the prophets have not your fathers persecuted? and they have slain them which shewed before of the coming of the Just One; of whom ye have been now the betrayers and murderers: (53) Who have received the law by the disposition of angels, and have not kept it. Now he flat accuses them of being stiffnecked and uncircumcised in heart. In other words, too hardheaded to listen to the truth. Obviously, he saw the crowd beginning to turn on him and knew the time of invitation had passed!
Also, no doubt, Stephen's choice of the word stiffnecked brought images of Exodus 32:7-10 and Exodus 33:5). Unlike far too many in the Body of Christ, the Jews knew their Scriptures. Shocked at how ignorant we are today.
Many commentators will say that Stephen's entire message was a defense, no, it was an indictment from beginning to end. Stephen had done exactly what Peter had done in Acts 5:28. Also, as Randy White points out, Stephen was not practicing evangelism as we know it in these verses either. It was pure condemnation of them and their actions. Of course, just like with the messages of Peter, had the nation repented, the Kingdom would have come.
Notice in v.53 where it says Who have received the law by the disposition of angels and have not kept it. In other words, even though God and the heavenly hosts gave you the Law, you still have not kept it. Again, nothing but condemnation. They lost it!
VERSES 54: When they heard these things, they were cut to the heart, and they gnashed on him with their teeth. As a result of Stephen's words, they became, as Albert Barnes puts it, enraged and indignant that he would dare accuse them of Jesus' death. Religion always produces a fleshly response. Understand, these had rejected the King and his Kingdom, there was nothing spiritual about them. They were responded in the flesh.
VERSE 55: But he, being full of the Holy Ghost, looked up stedfastly into heaven, and saw the glory of God, and Jesus standing on the right hand of God, Notice that this verse clearly show the Trinity.