"Though you already know all this, I want to remind you that the Lord delivered his people out of Egypt, but later destroyed those who did not believe." NIV translation
Now, Jude reminds us that our faith is an individual thing and not because we belong to a group. He refers to the people's rebellion (Numbers 14) where they were told to go in and take control of the promised land but did not want to because of fear. Because of their fear and disobedience, they had to wander in the desert until all that rebelled had died. Then, their children were led into the land by Joshua and Caleb the only two that had believed that God would give them the victory.
"And the angels who did not keep their positions of authority but abandoned their home - these he has kept in darkness, bound with everlasting chains for judgement on the great Day." NIV translation
Now, Jude gives us the example of the fallen angels (see Revelation 12:7-9) and how they were cast out of heaven. The angels are another example of the fact that God will not allow sin (disobedience) in His presence. They rejected God's authority as the Creator and were cast out.
"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." NIV translation
We are now reminded of the fate of Sodom and Gomorrah (Genesis 19) and how God rained down fire on them because of their sin. They were openly in rebellion against God and so were punished.
"In the very same way, these dreamers pollute their own bodies, reject authority and slander celestial beings." NIV translation
Jude has given us examples from the history of God's people and he now says the same things are happening. He refers back to the people he described in verse 4 who are changing God's grace into a license for immorality. That description is very true of our world today.