2-peter 2:5

Spared not the old world - The apostle's argument is this: If God spared not the rebellious angels, nor the sinful antediluvians, nor the cities of Sodom and Gomorrha, he will not spare those wicked teachers who corrupt the pure doctrines of Christianity.

Saved Noah the eighth - Some think that the words should be translated, Noah the eighth preacher of righteousness; but it seems most evident, from 1 Peter 3:20, that eight persons are here meant, which were the whole that were saved in the ark, viz. Shem, Ham, Japhet, and their three wives, six; Noah's wife seven; and Noah himself the eighth. The form of expression, ογδοον Νωε, Noah the eighth, i.e. Noah and seven more, is most common in the Greek language. So in Appian, Bell. Pun., p. 12, Τριτος δε ποτε εν σπηλαιῳ κρυπτομενος ελαθε, sometimes he the third (i.e. he with two others) lay hid in a cave. Andocides, Orat. iv. p. 295: Αἱρεθεις επι τουτῳ δεκατος αυτος, he himself the tenth (i.e. he and nine others) were chosen to this. See a number of other examples in Kypke.

World of the ungodly - A whole race without God - without any pure worship or rational religion.

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 awful 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 thrown in in the progress 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. This idiom is found, says Dr. Bloomfield, in the best writers - from Herodotus and Thucydides downward. See examples in Wetstein. The meaning in this place then is, that eight persons, and eight only 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. Compare the notes at Hebrews 11:7.

Bringing in the flood upon the world of the ungodly - Upon all the world besides 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.

2:5 And spared not the old, the antediluvian, world, but he preserved Noah the eighth person - that is, Noah and seven others, a preacher as well as practiser, of righteousness.Bringing a flood on the world of the ungodly - Whose numbers stood them in no stead.