51 “You stiff-necked and uncircumcised in heart and ears! You always resist the Holy Spirit; as your fathers did, so do you.
52 Which of the prophets did your fathers not persecute? And they killed those who foretold the coming of the Just One, of whom you now have become the betrayers and murderers,
53 who have received the law by the direction of angels and have not kept it.”
stiff necked (v.51) — obstinately disobedient, unwilling to bend — Exodus 33:5 (where Moses uses it of the Israelites)
The Jews boasted of circumcision as a sign of their favor with God and despised the uncircumcised Gentiles. But Stephen calls them uncircumcised in God’s sight (v.51) because their circumcision was just an outward ritual that did not reflect their inward state. Jeremiah 6:10; 9: 26; Ezekiel 44:7
resist (v.51) = fling yourself against in opposition
which of the prophets did your fathers not persecute (v.52) — 2 Chronicles 36:16; Matthew 5:12; 21:34-36; 23:35-37; Luke 13:34
killed (v.52) — 2 Chronicles 24:20-22
Just One (v.52) — Jesus Christ — Acts 3:14; Isaiah 11:4-5; 53:11. The Lord is just (justified, righteous) because only He perfectly fulfilled God’s law and will.
by the direction of angels (v.53) — Cf. Galatians 3:19; Hebrews 2:2, in which passages the angels are regarded as intermediaries through whom the Law was given; a view based, apparently, on Deuteronomy 33:2, where the LXX reads “At His right hand were angels with Him,” in lieu of “at His right hand was a fiery law unto them.” Cf. also Psalm 68:17. As the present verse is rendered in the text, a similar meaning is suggested …
The general idea would appear to be that additional luster was attached to the law on account of the presence and ministry of angels at its first promulgation. it’s inception was attended with all the concomitants of heavenly glory. — The Acts of the Apostles, by Thomas Walker, page 176
not kept it (v.53) — Stephen was accused of blaspheming the law. In his defense, he points out that it was his accusers who were in violation of the law.
That the rulers understood perfectly what Stephen had been getting at is clear from these verses and the rest of the account.
Apparently it became evident that the rulers would reject any appeal Stephen had hoped to make, and he sensed that they would not listen much longer, for suddenly the tone of his message changes. Rather than an appeal there is a stinging indictment. He seems to disown them as he changes his repeated “our fathers” to “your father,” and charges them with resisting the Holy Spirit, betraying and murdering Christ and despising Moses and the law which they pretended to uphold. In his indictment Stephen went backward from their sin against the Spirit, to that against Christ, to that against Moses and the law, for the effect it would have upon them. Actually their sin against the Spirit was that which sealed their doom. — Acts Dispensationally Considered, by C.R. Stam, ages 228-229.