This letter is a lot like the second chapter of 2 Peter. Jude also is warning the people against false teachers. This was a very severe danger because the apostles (who were eyewitnesses of Jesus’s ministry) were dying. False teachers were coming in and leading the people astray. Were all of these warnings (as also in Galatians, Colossians, 1 John and elsewhere) necessary? Why say the same things so many times? The answer is that the church really needed it. The proof of that is in the history of the church. Only a few decades later, some segments of the church started believing strange things not in the Bible. The history of the early church is largely a sad story of fighting heretics. This was Satan’s strategy for defeating God, since he failed to prevent Jesus from rising from the dead.