Verse 10. But these speak evil of those things which they know not. These false and corrupt teachers employ reproachful language of those things which lie wholly beyond the reach of their vision. See Barnes "2 Pe 2:12".

But what they know naturally. As mere men; as animals; that is, in things pertaining to their physical nature, or in which they are on a level with the brute creation. The reference is to the natural instincts, the impulses of appetite, and passion, and sensual pleasure. The idea of the apostle seems to be, that their knowledge was confined to those things. They did not rise above them to the intelligent contemplation of those higher things, against which they used only the language of reproach. There are multitudes of such men in the world. Towards high and holy objects they use only the language of reproach. They do not understand them, but they can rail at them. Their knowledge is confined to the subjects of sensual indulgence, and all their intelligence in that respect is employed only to corrupt and destroy themselves.

As brute beasts. Animals without intelligence. See Barnes "2 Pe 2:12".

In those things they corrupt themselves. They live only for sensual indulgence, and sink deeper and deeper in sensual gratifications.