Acts 7:51 - Matthew Poole's English Annotations on the Holy Bible

Stiff necked; a metaphor taken from heifers that are unaccustomed to the yoke. Uncircumcised in heart; such as had still depraved affections, which they ought to have put away rather than the foreskin of their flesh; for they were commanded to circumcise their hearts, Deuteronomy 10:16, which also God promised to do for his people, Deuteronomy 30:6. And St. Paul was not the first who spake of a twofold circumcision, Romans 2:28,29 but God looked always to the inward and spiritual part of his own ordinances, and men's observance of them. And ears; such as were not so much as willing to hear and know their duty. Ye do always resist the Holy Ghost, speaking by his prophets and ministers, and exhorting to true and serious piety: by this St. Stephen would abate their glorying in circumcision, which they so much boasted of, As your fathers did, so do ye: thus the prophet Ezekiel, Ezekiel 16:44, unto which may be here alluded, As is the mother, so is her daughter.