This is a reference to the cultic and moral sins of Eli’s sons (1 Samuel 2:12–17, 1 Samuel 2:22), as well as Eli’s weak rebuke of those sins (1 Samuel 2:23–25).1 Death was the penalty for showing contempt for the priesthood (Deuteronomy 17:12), blasphemy, and disobeying parents (Deuteronomy 21:18–21).2
13 And I declare to him that I am about to punish his house forever, for the iniquity that he knew, because his sons were blaspheming God, and he did not restrain them.