But these speak evil of those things which they know not. He means that they had no taste for anything but what was gross, and as it were beastly, and therefore did not perceive what was worthy of honor; and that yet they added audacity to madness, so that they feared not to condemn things above their comprehension; and that they also labored under another evil -- for when like beasts they were carried away to those things which gratified the senses of the body, they observed no moderation, but gorged themselves excessively like the swine which roll themselves in stinking mud. The adverb naturally is set in opposition to reason and judgment for the instinct of nature alone rules in brute animals; but reason ought to govern men and to bridle their appetites.

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. Notes, 2-Peter 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. Notes, 2-Peter 2:12.
In those things they corrupt themselves - They live only for sensual indulgence, and sink deeper and deeper in sensual gratifications.

Speak evil of those things which they know not - They do not understand the origin and utility of civil government; they revile that which ever protects their own persons and their property. This is true in most insurrections and seditions.
But what they know naturally - They are destitute of reflection; their minds are uncultivated; they follow mere natural instinct, and are slaves to their animal propensities.
As brute beasts - Ὡς τα αλογα ζωα· Like the irrational animals; but, in the indulgence of their animal propensities, they corrupt themselves, beyond the example of the brute beasts. A fearful description; and true of many in the present day.

(8) But these speak evil of those things which they know not: but what they know naturally, as brute beasts, in those things they corrupt themselves.
(8) The conclusion: These men are doubly at fault, that is, both for their rash folly in condemning some, and for their impudent and shameless contempt of that knowledge, which when they had gotten, yet nonetheless they lived as brute beasts, serving their bellies.

But these speak evil of those things which they know not,.... Which may more particularly refer to dignities, Jde 1:8; either angels, who are little known, and not at all, but by revelation, and yet were blasphemed, or evil spoken of by these men; either by ascribing too much to them, as the creation of the world; or by saying such things of them, as were below, and unworthy of them, as their congress with women, &c. or civil magistrates; these men were ignorant of the nature, use, and end, of magistracy and civil government, and so treated it with contempt; or the ministers of the Gospel, whose usefulness was not known, at least not acknowledged by them, and so became the object of their scorn and reproach: or it may refer more generally to the Scriptures, which false teachers are ignorant of, and yet speak evil of; either by denying them to be the Word of God, or by putting false glosses on them; and so to the several parts of the Scriptures, as to the law, the nature, use, and end of which they are not acquainted with; and therefore blaspheme it, by not walking according to it, or by denying it to be of God, and to be good, or by making the observance of it necessary to justification and salvation; and also to the Gospel, the doctrines and ordinances of it, which they speak evil of, despise and reject, not knowing the nature, value, and design of them:
but what they know naturally as brute beasts: man originally had a large share of natural knowledge, and there is in man still, notwithstanding the fall, by which his knowledge is impaired, a natural knowledge of God, and of things natural, civil, and moral; and there is a sensitive knowledge in man, which he has in common with the brutes, and which is here meant: and such was the brutish sensuality of these men, that
in those things they corrupt themselves; and act as brute beasts without shame and fear; yea, worse than brute beasts, as in the acts of unnatural lust, mentioned in Jde 1:7; whereby they corrupt both their souls and bodies, and so shall be destroyed, and perish in their corruption.

(2-Peter 2:12.)
those things which--Greek, "all things whatsoever they understand not," namely, the things of the spiritual world.
but what . . . naturally--Connect thus, "Whatever (so the Greek) things naturally (by natural, blind instinct), as the unreasoning (so the Greek) animals, they know," &c. The Greek for the former "know" implies deeper knowledge; the latter "know," the mere perception of the "animal senses and faculties."

But these - Without all shame. Rail at the things of God which they know not - Neither can know, having no spiritual senses. And the natural things, which they know - By their natural senses, they abuse into occasions of sin.