The Book of Jude is a wonderful passage that is a perfect prelude to the Book of Revelation.  It is a "wake-up call," reminding us of what happened in the early days of Christianity while preparing us for what will happen in these latter days.  When the Book of Jude was written, only 30 or so years had passed since the crucifixion and resurrection of Jesus Christ.  In that short period of time, Satan had already succeeded in corrupting the church to the point that error and heresy were rampant.  

Jude goes on to remind us that these things would happen in the last days.  

18 "In the last days there will be people who
don't take these things seriously anymore. They'll treat them like a joke, and make a religion of their own whims and lusts."  19 These are the ones who split churches, thinking only of themselves.  There's nothing to them, no sign of the Spirit!"  (MSG)  

With this passage, Jude is clearly saying that there are false teachers among us and even more will appear as the coming of Christ draws near.  While we must walk in love and be careful not to accuse any falsely, we need to take heed not to follow those that err from the Word of God.  Human men or women can become corrupt and teach falsely.  We as Christians in these latter days must be on guard for false doctrines which can so easily deceive us if we are not well-versed in the Scriptures.  We must speak out against any doctrine that is contrary to Word of God.  It is a real danger in Jude's eyes and he found it important enough to dedicate his writing to the topic.  I also find it important enough to stress the need for our dedication to God.  We must step away from our world of sin and self-satisfying pleasures and step into the fullness of God's Word.