54Now when they heard these things they were enraged, and they ground their teeth at him.
The grinding of teeth shows that the Sanhedrin now see Stephen as an enemy.1 They were enraged by what he has said (cf. Psalm 35:16; Luke 13:28).2 Notice they do not interrupt him at this point, but they are angry with the fact that he has identified them with those who have rebelled.
54 Now when they heard these things they were enraged, and they ground their teeth at him.