John Piper, in a sermon from Acts 7, The Story of a Stiff-Necked People:
So in Acts 7:1 the high priest gives Stephen a chance to defend himself, “Is this so?” he asks. And Stephen does a very strange thing. He tells a story—a condensed version of the history of Israel. He starts with Abraham at the beginning (in vv. 1–8). Then (in vv. 9–16) he dwells on Joseph and how the Israelites came to Egypt. Then he spends a long time on Moses (in vv. 17– 44). Then he closes with a brief reference to Joshua and David and Solomon (in vv. 45–50).
Finally, he draws his conclusion from this history. Acts 7:51–53: “You stiff-necked people, uncircumcised in heart and ears, you always resist the Holy Spirit. As your fathers did, so do you. Which of the prophets did not your fathers persecute? And they killed those who announced beforehand the coming of the Righteous One [Jesus], whom you have now betrayed and murdered, you who received the law as delivered by angels and did not keep it.”
So what was Stephen’s defense? He had been charged with speaking against Moses and the law, and against God and the temple. His defense is that history proves the opposite: it is Israel as a people that have stiffened their neck against God and resisted the Holy Spirit. They persecuted the prophets of God, and they killed Jesus the Son of God, and now they are about to kill a man “full of faith and the Holy Spirit.” They are the ones who need to give an account, not Stephen.