Jude writes to the remnant believers that they should earnestly contend for the faith which was once delivered unto the saints. Here again, we see the need for the remnant to know who is of God and who is not for Jude writes that ungodly men have crept in unawares. They will be surrounded by ungodly men and therefore, they will need to keep the words of the apostles of the Lord Jesus Christ in remembrance.
(Jude 11 KJV) Woe unto them! for they have gone in the way of Cain, and ran greedily after the error of Balaam for reward, and perished in the gainsaying of Korah.
Jude tells them that the ungodly will receive the judgement of God.