But these people blaspheme all that they do not understand, and they are destroyed by all that they, like unreasoning animals, understand instinctively. Woe to them! For they walked in the way of Cain and abandoned themselves for the sake of gain to Balaam's error and perished in Korah's rebellion.
The thrust of Jude's little letter is to persuade the church to reject false teachers. The brief description of false teachers and their resultant blasphemy is a reminder that should still warn us today. False teachers buy into and also sell swarmily an easy, intuitive theology. They want things easy, as is evidenced by Jude deriding them as "unreasoning animals". They are destroyed by their lack of thinking.
When Jude says they blaspheme by following their instincts, he is making it clear that by do doing, their doctrines are rooted in sin and error. It is our sinful natures that are instinctive to us. God calls us to apply ourselves to thoughtful belief and actions based on His revelation that comes from outside us, not from within. But false teachers always claim to have an easier route to salvation and a happy life. They will twist God's revelation, but always with an instinctive bend towards sin. They are unreasoning animals. They thus avoid biblical theology altogether.
These practices lead false teachers to emulate three of the Old Testament's really bad blasphemers. They are like Cain, who disobeyed God by not offering his best sacrifice, and when confronted chose to rebel against the revealed truth by killing his own brother. They are like money-hungry Balaam who sold his prophetic talents to the highest bidder with no regard for God's holiness or the truth. They are like Korah who defied Moses and God's leadership and led a portion of Israel into his rebellion, only to have the whole lot of them be swallowed up alive in a sinkhole that bottomed out straight to hell.
So Christians need to be on guard. False teaching is so easy, and often so attractive. Biblical theology is rewarding but requires diligence and sometimes mental effort. Always be cautious of the broad and easy road, of any "Christian" teacher who claims to have found something nobody has every seen or taught before. Beware of sermons that contain "secrets" for living. Jesus said this easy road leads to destruction, and his half-brother Jude agrees with Him, reminding us that their are on-ramps to that broad and dangerous road even from the church.