Jude 1:11

Woe unto them! for they went in the way of Cain, and ran riotously in the error of Balaam for hire, and perished in the gainsaying of Korah.

Glancing above at the preceding verses, it will be noted that Jude gave three examples of apostasy: the ancient Israelites, the rebel angels, and the wicked cities of Sodom and Gomorrah (Jude 1:1:5-7). Next, he indicted the licentious Gnostic heretics as just as guilty as the ancient apostates, emphasizing their sinful and ignorant behaviour by contrasting it with the restraint of the archangel Michael (Jude 1:1:8-9), these two verses being somewhat of a parenthetical note. He returned to the indictment of the wicked men in Jude 1:1:10; but in this (Jude 1:1:11), he equates and compares their conduct with the wickedness of three of the worst Old Testament apostates: Cain, Balaam, and Korah.

Cain ... Balaam ... Korah ... Jude assumed that his readers were thoroughly familiar with the Old Testament events associated with these three names of infamy; and we shall do our readers the courtesy of making the same assumption with regard to them. For those who would like to "brush up a bit," the narratives of Cain (Genesis 4), Balaam (Numbers 22-24), and Korah (Numbers 16) are among the most interesting records in Scripture. Jude's reason for the choice of these three examples might have been lodged in the spectacular punishments they received. Cain was cursed of God; Balaam was found dead with God's enemies fighting against Israel, and Korah was swallowed up by an earthquake that split open the earth, taking Korah and all of his company to their death. The lesson is that a grievous punishment will be meted out to wicked men. Another reason for the selection of these three was also probably that of their sins being similar to the sins of the wicked Gnostics. Like Cain, they were innovators with a fierce hatred of any who rejected their ideas. Like Balaam, they were greedy, covetous, and willing to do any dishonorable thing whatever for the sake of money. Like Korah, they rebelled against God's appointed authorities, the sacred apostles, prophets and teachers of the new covenant, just as Korah had rebelled against Moses. Still another possible purpose in using the example of Cain derived from the need to refute the Ophites, called also Cainites. Bruce has this on that evil system:

In the early days of Christianity there was one heretical (Gnostic) group which actually venerated Cain and his successors as champions of right, and claimed to be akin to him "and to the men of Sodom and Esau and Korah" (as Epiphanius informs us)[36]

There is further discussion of this above, under 1 John 3:12, where Cain was cited as an example of wickedness. Also see above, under 2 Peter 2:15, where Balaam was similarly cited. Apparently, all three of these, Cain, Balaam, and Korah were considered to be especially notorious sinners, and frequently referred to as examples and warnings.