John Calvin Commentary Isaiah 64:2

"as when fire kindleth the brushwood, [and] the fire causeth the waters to boil; to make thy name known to thine adversaries, that the nations may tremble at thy presence!" — Isaiah 64:2 (ASV)

As by the burning of a melting fire, the fire has made the water to boil. All this might be read either in the future or in the subjunctive; as if he had said, “O Lord, if you came down, the nations would tremble at your presence; your enemies would instantly be melted away.” But I think that the translation which I have given is more simple; for it is very certain that the Prophet here alludes to Mount Sinai, where the Lord openly revealed himself to the people. From this we also see the gross absurdity of the division of this chapter, since those events are related in support of that prayer which should rather have been placed at the beginning of the chapter.

We have previously seen that the prophets, when they relate that God assisted his people, bring forward an instance in the history of redemption. Whenever, therefore, the prophets mention this history, they include all the benefits that were ever bestowed by God on his people; not only when he delivered them from the tyranny of Pharaoh, when he appeared to them in Mount Sinai, but also when, during forty years, he supplied them with all that was necessary in the wilderness, when he drove out their enemies, and led them into the possession of the land of Canaan. In a word, they include all the testimonies by which he previously proved himself to be gracious to his people and formidable to his enemies.

He says that the melting fire made the waters boil, because, contrary to custom, fire and lightning were mingled with violent showers; as if he had said that the fire of God melted the hardest bodies, and that the waters were consumed by its heat. To the same purpose is what he adds, that the mountains flowed at his presence; for he opened up a passage for his people through the most dreadful obstacles.