Jude 1:5 KJV
I will therefore put you in remembrance, though ye once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.
Jude saw even in His day certain men creeping in and destroying (or attempting to destroy) the ministry of God. His conclusion was that we must contend for the faith and he did so:
By reminding them of the judgment of God
Jude lists those who had experienced that judgment in the past.
By reminding them of the warning of God
That there would be those false prophets and mockers of what is godly that should come.
By urging them to be built up in the faith
This one thing is so necessary and yet so lacking in the Christian world today. We have heard the warning yet, just like Peter, we are asleep when we should be praying. We are careless about spiritual things when we should be the most concerned about those things.
He appealed to the Lord
Who is the One who is able to keep us from falling.
To Him alone we look.