Apostates promise spiritual life but are empty clouds which bring the hope of rain, but actually deliver nothing but dryness and death. They preach a false gospel that leads only to hell.
Commentary from the MacArthur study Bible, notes for Jude 12.
The apostles had warned the coming generation about apostates, so that they would be prepared and not be taken by surprise. God’s Word is designed to warn and protect; as verse 18 indicates, there had been continually repeated warnings.
Commentary from the MacArthur study Bible, notes for Jude 17.