skleruno describes becoming or making resistant, especially in warnings not to resist the voice heard.

The attested usage describes a resistant condition or its causing: Hebrews repeatedly warns hearers not to make their hearts resistant when they hear the voice, while Acts describes people becoming resistant and Romans places the action within a statement about mercy and will.

1. Becoming or making the heart resistant · dominant

   In Hebrews 3:8, Hebrews 3:15, and Hebrews 4:7, the active aorist subjunctive occurs in the warning, “do not harden your hearts,” when the voice is heard. Hebrews 3:13 speaks of someone who “may be hardened by the deceit of sin,” Acts 19:9 describes people who “became hardened and disobedient” while speaking against the Way, and Romans 9:18 states, “he hardens whomever he wills.”

   Hebrews 3:8Hebrews 3:15Hebrews 4:7Hebrews 3:13Acts 19:9Romans 9:18

In the Anselm Project Bible: The APB uses “became hardened” and “may be hardened” for passive forms, and “do not harden your hearts” and “he hardens” for active forms. These renderings reflect the clauses as translated.