What does Stephen mean by uncircumcised in heart and why did Israel lack it?
Stephen's phrase 'uncircumcised in heart' refers to lacking the spiritual heart circumcision that God performs when a person believes, which is salvation's requirement in every dispensation. Israel lacked it because they knew and kept only physical circumcision, not the inward work of belief.
Les explains that Stephen rehearses Israel's history—Abraham, Isaac, Jacob, Moses, David, and Solomon—to drive home that the Messiah they crucified was the promised Son of God, the King of Israel, and the Son of David. Then in verse 51 Stephen calls them stiff-necked and uncircumcised, not in the flesh, because they certainly kept that. What they lacked was heart circumcision, which is that part of salvation in any dispensation: when a person believes, God executes a spiritual circumcision. Israel knew nothing of that, though they knew the physical.
Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye.
More Questions
Why did Stephen call them uncircumcised if they had physical circumcision?
When Stephen called them "uncircumcised," he did not mean physical uncircumcision, because they had kept physical circumcision; he meant they lacked heart circumcision.
In what way were Stephen's hearers circumcised yet uncircumcised?
Israel was physically circumcised but lacked heart circumcision, which is the spiritual circumcision that accompanies believing.
When does God perform spiritual circumcision?
When a person comes to believe, God performs or executes a spiritual circumcision.