Only the book of Jude records the account of Michael and Satan disputing over the body of Moses. The account likely came from the extra-biblical work Ascension of Moses. That writing was not inspired, but because Jude used it, we can be certain that the part he recorded is true.
Just like we don’t know the source of the story, we also don’t know why the dispute happened. Since Jude left that part out, we can only speculate. The Ascension of Moses implies that Satan was accusing Moses of sin, not worthy of either burial or resurrection, and that may have been the reason.
Rather than speculate, we should focus on why Jude included the account. He was contending against false teachers who were, in some manner, trying to control or curse (“revile”) “angelic majesties” (1:8). Jude used this account to show that one of the greatest angels with insight into the spiritual realm didn’t even do that, leaving Satan’s judgement up to God.
False teachers in Jude’s time had stealthily crept into the church (Jude 1:4b), teaching that God’s grace meant people could sin as much as they wanted (Jude 1:4b) and denying the reality of Jesus (Jude 1:4c). Those false teachers apparently were also claiming to do some form of spiritual battle. Jude used the account of Michael and Satan to illustrate how even one of the greatest angels did not curse Satan, and that, therefore, the false teachers were like dumb animals (Jude 1:10).
Today, the church is filled with false teachers. Some, like those in Jude’s day, have made a name for themselves by claiming to bind Satan and his demons. Other teachers are overtly engaged in sinful practices by presuming on God’s grace. They focus on material wealth, being accepted by the world, and some are even openly engaged in premarital sexual relationships. In each case, they are teaching others to do the same.
Therefore, Jude’s warning from the first century is just as relevant today, as are his commands on how to not become deceived. First, we are to continue to build ourselves up by reading Scripture (Jude 1:20a), keeping in prayer (Jude 1:20b), obeying God (Jude 1:21a), and waiting for Jesus’ return (Jude 1:21b). This is so that we can tell the difference between true and false teachers.
Second, we need to have mercy on those being deceived by the false teachers, “snatching them out of the fire” (Jude 1:22–23a). While false teachers were “long beforehand marked out for this condemnation” (Jude 1:4), there is still hope that people listening to them can be saved.
Third, even as we try to rescue some, we are to keep a wide berth from the false teachers and to be very careful with those who have been deceived to keep from becoming polluted, ourselves (Jude 1:23b).