What does Acts 7:51 mean?

One of Stephen's arguments addresses the accusation that he spoke words against the Law (Acts 6:13) and that Jesus preached that Jews did not have to follow the Mosaic law (Acts 6:14). Jesus, of course, preached against the man-made oral law (Matthew 23) and deeply respected the Mosaic law (Matthew 5:17–19). But here, Stephen gets further into his argument, showing that the Jewish leaders who claim to uphold the sanctity of the Law are descendants of those who rejected God's messengers.

Circumcision was the ritual God gave Abraham to indicate that God chose Abraham to be the patriarch of His people. Abraham would have many descendants, and his people would own the land of Canaan and bless the world (Genesis 17:1–14). To be uncircumcised was to reject one's place in God's covenant. Stephen has already mentioned that Abraham, Isaac, Jacob, and Jacob's sons were circumcised, taking their place in the Abrahamic covenant. In the same way, the men of Stephen's audience have been circumcised and expect to take their place in God's promise to Abraham.

But Stephen attacks their hypocrisy. They may be circumcised physically, but neither their hearts nor their ears are attuned to God. Instead of following God's leading in their hearts through the Holy Spirit, they actively resist Him (John 16:8).

Stephen's accusers and the members of the Sanhedrin are just like their fathers. Weeks after the Israelites escaped Egypt, while Moses was on Mount Sinai getting the Law that would teach them how to be God's people, the Israelites built a golden calf and worshiped it as their rescuer (Exodus 32). Before Joshua died, he charged the Israelites to choose whom they would serve. They vowed to serve God. Joshua told them they wouldn't (Joshua 24:16–20). He was right. Before and after the account of Joshua's death, Judges 2 talks about the Israelites' disobedience. They couldn't even wait for Joshua to die. It's no wonder Stephen's audience is not faithful to the God they claim to serve.