And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly; 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.