Jude wanted to write about their common salvation but felt compelled to write to address false teachers that have penetrated the community and teaching heresy.
5-19 God’s promise of judgment of the ungodly
5-7 Historical precedent
He recalls three examples of gods judgment as a warning to these false teachers. We are all prone to spiritual amnesia so we must give attention to what God has done in the past. (Ie testimony; history repeats itself; open theism). They knew it already, a reference to OT and probably the faith handed down (maybe even 2 Peter). They do not need new revelation to contend as much as they need to remember and apply what they already know and have read. Remembering Gods word and gods work in history, both blessing and judgment is necessary for perseverance. Scripture is meant top be meditated on and committing to memory to be useful throughout the day. This is an argument against the false teachers who are teaching new revelation. Knowing scripture is a means of grace God uses to cause us to persevere. These examples have two goals: give assurance to those who are being kept and give resolve to those who are wavering.
The first example is the people of Israel. God saved them out of bondage in Egypt and later destroyed those who literally did not continue in belief, they apostatized. Jude’s point is that even if one is part of the believing community, if they turn away for whatever reason (unbelief, heresy, etc) they lose their hope. The same God that saved them destroyed them for their unbelief; both the saving and the destroying are manifestations of God’s power. The NT tells us that not all Israel (physical) is Israel (spiritual) (Rom 9:6-8).
Second are the angels who fell or apostatized and are now kept awaiting judgment (not all angels are elect 1 Tim 5:21). Can This can be a reference to the original fall of angels or the sons of God in Gen 6:1-4. They left their positions of their own choice because they were not satisfied with their God given position. They are bound in chains so that they cannot move; in darkness so they cannot see; and therefore cannot unite. Not all are bound, some still roam actively (Luke 8:31; Rom 8:38; Col 2:15). Gen 6 is Satans plot to usurp the promise of God in Gen 3:15.
Third example is Sodom and Gomorrah who were judged for their sexual immorality. Sodom and Gomorrah in Gen 13:10 was part of the Sidon Valley that is described as well watered but because of the sin of Sodom and Gomorrah God rained down sulfur on it and to this day it is barren (volcano). Their punishment is suffering presently. Lk. 16:19-31. Have you ever thought about how people in hell will be sustained for all eternity? God will supernaturally sustain body and soul for their just punishment. What is common to all three examples: rebellion; sexual immorality; corporate departure
8-13: Parallel: characteristics of present ungodly is same
Based upon the three examples the false teachers deserve the same. In like manner the false teachers have clear warnings but their arrogance has blinded them of history and truth.