Heresy does not begin in the head with doctrines, it begins in the heart with the love of sin. It begins in the heart and it moves to the head. Every heretic in the history of the world has loved his/her sin and loved unrighteousness, and would not repent. And so they twisted the true and precious doctrines of Christianity. I think what Jude is saying is that these men, who had crept in, did that—they twisted the grace of God so that they would not have to repent from their sin, and so they denied Jesus Christ and became intellectual heretics.1
Nick Batzig
4 For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.