Jude 6-7

And the angels who did not keep their positions of authority but abandoned their proper dwelling--these he has kept in darkness, bound with everlasting chains for judgment on the great Day. In a similar way, Sodom and Gomorrah and the surrounding towns gave themselves up to sexual immorality and perversion. They serve as an example of those who suffer the punishment of eternal fire. 

these he has kept in darkness, bound with everlasting chains for judgment on the great Day 
2 Peter 2:4-9  For if God did not spare angels when they sinned, but sent them to hell, putting them in chains of darkness to be held for judgment; if he did not spare the ancient world when he brought the flood on its ungodly people, but protected Noah, a preacher of righteousness, and seven others; if he condemned the cities of Sodom and Gomorrah by burning them to ashes, and made them an example of what is going to happen to the ungodly; and if he rescued Lot, a righteous man, who was distressed by the depraved conduct of the lawless (for that righteous man, living among them day after day, was tormented in his righteous soul by the lawless deeds he saw and heard) --if this is so, then the Lord knows how to rescue the godly from trials and to hold the unrighteous for punishment on the day of judgement.

In a similar way Sodom and Gomorrah 
Matthew 10:15  "Truly I tell you, it will be more bearable for Sodom and Gomorrah on the day of judgment than for that town."

and the surrounding towns 
Deuteronomy 29:23-25  The whole land will be a burning waste of salt and sulfur--nothing planted, nothing sprouting, no vegetation growing on it. It will be like the destruction of Sodom and Gomorrah, Admah and Zeboyim, which the LORD overthrew in fierce anger. All the nations will ask: "Why has the LORD done this to this land? Why this fierce burning anger?" And the answer will be: "It is because this people abandoned the covenant of the LORD, the God of their ancestors, the covenant he made with them when he brought them out of Egypt."

They serve as an example of those who suffer the punishment of eternal fire 
Matthew 25:41  "Then he will say to those on his left, 'Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.' "
2 Peter 3:7  By the same word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of the ungodly.

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984, 2011 by Biblica, Inc.® Used by permission. All rights reserved worldwide. 
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