John Calvin Commentary Isaiah 66:16

"For by fire will Jehovah execute judgment, and by his sword, upon all flesh; and the slain of Jehovah shall be many." — Isaiah 66:16 (ASV)

For Jehovah will judge in fire. Here he brings forward nothing new, but merely confirms the former statement, and shows that this judgment will be dreadful, so that no one may think that it is a matter of small importance. Accordingly, he describes that horror in strong language, that the wicked may fear, and that believers, on the other hand, may keep themselves holy and chaste, and may withdraw from the society of the wicked. Yet let them endure patiently the unjust and cruel attacks of enemies, until the armed avenger comes forth from heaven.

And many shall be the slain of Jehovah. He threatens the destruction of all people, so that there may be a prodigious mass of corpses. And he expressly added this, because ungodliness reigned everywhere, and believers were subjected to a dangerous temptation because of the prosperity of the wicked; for such is the fickleness of our minds, that we allow ourselves to be led away by a bad custom, and are alarmed by a multitude, as if it were sufficient for restraining the hand of God. This sinful fear the Prophet corrects by reminding them, that the more ungodliness will abound, and the greater the number of wicked men, so much the more will the wrath of the Lord be kindled, so that he may make a greater and more extensive slaughter; and the multitude and conspiracy of the ungodly shall not hinder him from carrying them away by the same ruin.