Albert Barnes Commentary Acts 7:51

"Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Spirit: as your fathers did, so do ye." — Acts 7:51 (ASV)

You stiff-necked. Stephen's discourse has every appearance of having been interrupted by the clamors and opposition of the Sanhedrin. This verse has no immediate connection with the one that precedes it and appears to have been spoken in the midst of much opposition and clamor. If we may conjecture in this case, it would seem that the Jews saw the drift of his argument, interrupted him, and that when the tumult had somewhat subsided, he addressed them in the language of this verse, showing them that they possessed a character precisely similar to their rebellious fathers.

The expression stiff-necked is often used in the Old Testament (Exodus 32:9; Exodus 33:3, 5; Exodus 34:9; Deuteronomy 9:6, 13; Deuteronomy 10:16, and so on). It is a figurative expression taken from oxen that were unruly and would not submit to being yoked. Applied to people, it means that they were stubborn, stubbornly disobedient, and unwilling to submit to the restraints of law.

Uncircumcised in heart. Circumcision was a sign of being a Jew—of acknowledging the authority of the laws of Moses. It was also emblematic of purity and of submission to the law of God. The expression uncircumcised in heart denotes those who were not willing to acknowledge the law and submit to it. They had hearts filled with vicious and unsubdued affections and desires.

And ears. This refers to those who are unwilling to hear what God says. Compare to Leviticus 26:41; Jeremiah 9:26; Romans 2:28–29.

Resist the Holy Spirit. You oppose the message that is brought to you by the authority of God and the inspiration of His Spirit. The message brought by Moses, by the prophets, by the Savior, and by the apostles—all by the infallible direction of the Holy Spirit—they and their fathers opposed.

As your fathers did, and so on. As he had specified in Acts 7:27, 35, 39-43.