John Calvin Commentary 2 Peter 2:5

"and spared not the ancient world, but preserved Noah with seven others, a preacher of righteousness, when he brought a flood upon the world of the ungodly;" — 2 Peter 2:5 (ASV)

The old world. The meaning of what he says is that God, after having drowned the human race, formed again, as it were, a new world. This is also an argument from the greater to the less; for how can the wicked escape the deluge of divine wrath, since the whole world was once destroyed by it? For by saying that only eight were saved, he intimates that a multitude would not be a shield against God to protect the wicked, but that as many as sin shall be punished, whether they are few or many in number.

But it may be asked why he calls Noah the preacher of righteousness. Some understand that he was the preacher of the righteousness of God, since Scripture commends God's righteousness because he defends his own and restores them, when dead, to life. But I rather think that Noah is called the preacher of righteousness because he labored to restore a degenerated world to a sound mind, not only by his teaching and godly exhortations but also by his anxious toil in building the ark for the term of one hundred and twenty years. Now, the Apostle's purpose is to set before our eyes God’s wrath against the wicked, so as to encourage us at the same time to imitate the saints.