The high priest asked Stephen if the accusations levelled against him were true, but he used the slot he could have used to defend himself to tactically preach the gospel. Until we die to self, we're not yet ready to represent Christ. Having observed protocols, he secured the attention of the council; when we disregard protocols, our message will also be disregarded. Then he proceeded by tracing all of them back to Abraham; always establish a common ground before introducing the uncommon. If he'd started with the gospel of Christ, they'd have shut him down immediately.