Albert Barnes 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)

And spared not the old world. The world before the flood. The argument here is that He cut off that wicked race, and thus showed that He would punish the guilty. By that dreadful act of sweeping away the inhabitants of a world, He showed that people could not sin with impunity, and that the incorrigibly wicked must perish.

But saved Noah the eighth person. This reference to Noah, like the reference to Lot in 2 Peter 2:7, seems to have been included in the course of the argument as an incidental remark, to show that the righteous, however few in number, would be saved when the wicked were cut off.

The phrase "Noah the eighth" means Noah, one of eight; that is, Noah and seven others. Dr. Bloomfield notes that this idiom is found in the best writers—from Herodotus and Thucydides onward (see examples in Wetstein).

The meaning here, then, is that eight persons, and only eight of that race, were saved, thus showing that while the wicked would be punished, however numerous they might be, the righteous, however few, would be saved.

A preacher of righteousness. In Genesis 6:9, it is said of Noah that he was a just man and perfect in his generations, and Noah walked with God; and it may be presumed that during his long life he was faithful in reproving the wickedness of his age and warned the world of the judgment that was preparing for it .

Bringing in the flood upon the world of the ungodly. Upon all the world except for that pious family. The argument here is that if God would cut off a wicked race in this manner, the principle is settled that the wicked will not escape.