John Calvin on Deuteronomy 9:17

17. And I took the two tables, and cast them out Moses here accuses himself of no transgression; he does not, therefore, give us to understand that he was urged to break the tables by the impetuosity of excessive anger; but rather he again repeats what they had deserved, and consequently that he discharged the office of a herald, Lat. “Fecialis munere;” alluding doubtless to the custom of the Roman Feciales, in throwing a bloody spear into the territories of others as a declaration of war. See Liv. 1:32. so as to denounce, not by word of mouth only, but by a solemn rite also, that God’s Covenant was broken and made void by their perfidiousness. For which reason also he cast down and broke the tables before their eyes, in order that being alarmed by so awful a punishment, they might more earnestly betake themselves to the expiation of their sins.