What does Jude 1:9 mean?
In the previous verse, Jude referred to those who "blaspheme" celestial beings, including angels. Here, he cites the example of Michael the archangel. Michael is presented in Scripture as the chief angel in combating the Devil (Daniel 10:13, 21; 12:1; Revelation 12:7–9). In this incident, Michael refused to usurp God's authority, which stands in contrast to the audacity of false teachers to slander authorities and angels.This story comes from Jewish tradition, as recorded in a non-Scriptural book titled The Assumption of Moses. In the context Jude is using, it doesn't really matter whether this is an actual, historical event, or merely an example being given from a traditional story. Jude's reference does not imply that The Assumption of Moses is infallible, only that this was a story known to his readers. The key point here is that Michael, despite being a powerful being himself, refused to slander the Devil. This proves the arrogance of the apostates Jude is discussing. They are so conceited that they're doing what even an archangel won't!
Moses, Israel's leader during the exodus from Egypt, was buried by the Lord in an unknown place (Deuteronomy 34:1–6). According to the traditional account, Michael and the Devil had engaged in a dispute about the burial of Moses' body, but Michael would not slander the Devil. Instead, he yielded to God's authority by trusting Him to rebuke the Devil. We do not know the details of the dispute, but God may have assigned Michael with the task of guarding Moses' grave, and the Devil may have wanted to steal Moses' body to make it an object of worship.