Stiff-necked; hard of neck, inflexible. The word σκληροτράχηλος only occurs here in the New Testament. But it answers in the LXX. to the Hebrew פרֶעֹאהשֵׁקְ (hard of neck); see Exodus 33:3, Exodus 33:5, and elsewhere.
In applying this expression to his hearers, Stephen was using the identical language of Moses when he conveyed God's rebuke to them. Considering that they professed to be standing on Moses' side against Stephen, this must have made his words doubly cutting to them.
Uncircumcised in heart; ἀπερίτμητος only occurs here in the New Testament, but it is found in 2 Macc. 1:51; 2:46; and in the LXX. of Exodus 12:48; 14:3; 1 Samuel 17:26, and elsewhere for the Hebrew לרֵעֹ.
The word, in its application to his Jewish audience, contains a whole volume of rebuke. They prided themselves on their circumcision, they trusted in it as a sure ground of favor in the sight of God; but all the while they were on a level with the heathen whom they despised, and were to be reckoned among the uncircumcised whom they loathed.
For they were without the true circumcision, that of the heart. Here again Stephen was teaching in the exact spirit and even words of Moses and the prophets. See Leviticus 26:1-46. 41; Deuteronomy 10:16 (where Stephen's two reproaches occur together); Jeremiah 9:26; Ezekiel 44:7; and many other passages.
Compare the teaching of St. Paul (Romans 2:28, Romans 2:29; Philippians 3:2, Philippians 3:3; Colossians 2:11; and elsewhere).