Question: "What is the Assumption of Moses?"
Answer: The Assumption of Moses is a book, dated to the first century, supposedly relating prophecies told to Joshua by Moses. The book is sometimes referred to as the Testament of Moses. Its contents are referred to by several of the early church fathers, including Origen, but the book was not and is not considered a part of the biblical canon. Unlike the Bible, the Assumption of Moses is poorly preserved, existing in only one manuscript, translated into Latin, which is dated from after AD 500 and is missing a large portion of the text.
Even though the Assumption of Moses is nearly lost and clearly of a late date, it does have some connection to modern biblical scholarship. The fact that the Assumption of Moses is mentioned—though not canonized—by early Christians makes it historically interesting. It is also possible that Jude alludes to an incident in the Assumption of Moses when he mentions Michael and Satan disputing over the body of Moses (Jude 1:9). The story in Jude matches the traditional Jewish story, which is likewise related in the Assumption of Moses. This means the first-century work is not the origin of that story, but another telling of it.
Jude’s citation of the Assumption of Moses—if, in fact, he was citing that particular work—is not necessarily an endorsement of the work itself. Paul, for example, cited a non-Christian poet in Acts 17:28 and a non-scriptural narrative in 2 Timothy 3:8. Both references were meant to support a particular point Paul was making. Similarly, Jude’s possible allusion to the Assumption of Moses was to help further his point about false teachers.