Why were Michael and Satan arguing about the body of Moses?

Answer:

"Yet in the same way these men, also by dreaming, defile the flesh, and reject authority, and revile angelic majesties. But Michael the archangel, when he disputed with the devil and argued about the body of Moses, did not dare pronounce against him a railing judgment, but said, "The Lord rebuke you!"" (Jude 8-9).

This passage has led to numerous speculations over the centuries because the event is not recorded in the Old Testament; yet, Jude refers to it as a piece of history. Some try to make it symbolic, but there is no reason to think that Jude is using figurative language in this text.

Most likely the time being discussed is right after the death of Moses. "So Moses the servant of the LORD died there in the land of Moab, according to the word of the LORD. And He buried him in the valley in the land of Moab, opposite Beth-peor; but no man knows his burial place to this day" (Deuteronomy 34:5-6). The fact that God had Moses' body buried does hint that there was a concern.

The most logical guess that I've run across is that people have a tendency to idolize things. The Israelites turned the bronze serpent into an idol (II Kings 18:4). We see the Catholic church venerating anything that hints of being an ancient religious relic, including claiming to have bones of apostles or religious men. Therefore, it is not hard to imagine that Satan wanted access to the body of Moses to corrupt God's people in some manner. God prevented this from happening by keeping the location of Moses' body secret, but I'm sure Satan protested this.

However, this remains a speculation. Instead of getting sidetracked, we need to focus on the point that Jude was illustrating: No one should sit in judgment over the fate of another person, even when we know the ultimate fate of someone like Satan. God is the Judge of all the world and we must respect His authority in these matters.