For by fire will the LORD execute judgment repeats the threat mentioned in the previous verse. The Hebrew particle ki rendered For may be a logical connector (see the first example below) or an emphatic marker (see the second example below). The Hebrew verb translated execute judgment generally refers to passing judgment on a person, but here it means “punish” (Good News Translation). The form of this Hebrew verb is unusual, but it probably implies that God will use fire as the means by which he will punish those who are evil.

And by his sword, upon all flesh uses another metaphor to describe the LORD’s punishment of his enemies. The image of the sword always implies an attack by a foreign nation (see 1.20). His sword means God will use a foreign nation as an agent to carry out the punishment. The mention of his sword seems to conflict with earlier promises that no foreigner will ever invade Judah again (see 52.1; 60.10-14, 18). This punishment will fall upon all flesh, that is, upon every human being. Good News Translation adds the implied information “whom he finds guilty,” but it is better not to make this explicit. However, the following footnote could be added: “This punishment is not meant for all the inhabitants of Jerusalem, but only for those practice Canaanite idol worship.”

And those slain by the LORD shall be many: The LORD will kill many people. Not everyone will die, but many will die by fire or the sword. God’s judgment will distinguish between the many who will be saved, and the many who will be punished. Good News Translation says “and many will be put to death,” which implies that Yahweh will use an agent to do this.

For the translation of this verse consider the following examples:

• … because it is with fire that the LORD will punish,
and with his sword he will punish everyone.
There will be many who are killed in this way by the LORD.

• Indeed, the LORD will punish by fire,
and by his sword he will punish all people.
Many will die because of it.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .