John Calvin Commentary Deuteronomy 9:3

"Know therefore this day, that Jehovah thy God is he who goeth over before thee as a devouring fire; he will destroy them, and he will bring them down before thee: so shalt thou drive them out, and make them to perish quickly, as Jehovah hath spoken unto thee." — Deuteronomy 9:3 (ASV)

Understand therefore this day. He concludes from what has preceded that the Israelites would be too perverse unless they acknowledge that their enemies were overcome by the hand of God. And, to heighten the miracle even more, he uses an analogy, comparing God to a fire that consumes so many nations in an extraordinary and incredible manner.

It is as if he had said that it could not be brought about by human or ordinary means that so many and such warlike peoples could perish so quickly. Elsewhere God is called a consuming fire in a different sense, that we may fear His wrath and power; but here Moses only means that the destruction of the Canaanite nations was His wonderful work.