Jude 5b

Jude's next phrase, right before getting into the Old Testament examples, is "though ye once knew this". This could mean that his beloved readers have simply already learned it and he is applying it for this specific purpose. Or it could be that the readers have learned it but have all but forgotten it, and need the reminder so that they don't fall into the trap of apostasy that the false teachers are laying. Regardless, it can teach us something today.
The application: First Jude mentions that they "once knew this". This tells us that at the very least, his readers had a working knowledge of the scriptures. In order to be able to properly apply anything to the dark times of today, we must know God's Word. Second, we must be willing to continue. Jude put them in remembrance, but we must consistently put ourselves in remembrance of who God is, what He has done, and what we are learning. Then we must be willing to contend. While it is unlikely that all his readers heeded this call, we have a copy of the Word of God and convictions to go with it, so it is obvious that some did. We must listen to Godly counsel and follow in the steps that God lays out. Paul gives similar instructions in his second letter to Timothy (II Tim. 3:10-17). It was of utmost importance to these men of the New Testament, inspired by God, that our faith be learned, kept, and contended for, and it should be no different for us today.