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Jude's entire book is based on Matthew 7:15-20, where Jesus tells us, "Beware of false prophets," and "by their fruits you will know them." We should keep this thought in the back of our minds as we study Jude because most of the book describes these false teachers and their false teachings. Jude is giving this warning so that we will be able to spot them when they come out, when they begin to show their fruit.
In this way, Jude and II Peter are both witnesses to the certainty of false teachers, giving us instruction on identifying them and their effects. That said, however, the two epistles are only similar on the surface. They bring out different nuances of these false teachers. It is good to read them together, but it is also good to study them separately, because they are not necessarily saying the exact same things. They agree, but they give us different details, different information, so we can know more fully how to spot these false teachers.
Richard T. Ritenbaugh