2 Peter 3:16

2 Peter 3:16 as also in all his epistles, speaking in them of these things; wherein are some things hard to be understood, which the ignorant and unstedfast wrest, as they do also the other scriptures, unto their own destruction.

Expanded Translation

as also is true in all his epistles (letters), speaking in them concerning these very things (of which I have just spoken). In these epistles certain things (statements) are hard to understand, which the unlearned, (uninstructed, ignorant) and unstedfast (unsettled, unstable) wrench, distort, and pervert, as they do also the other scriptures, unto their own destruction, ruin, and damnation.

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as also in all his epistles, speaking in them of these things

The letters of Paul, particularly those addressed to the Thessalonians, say much concerning the very subjects treated in this epistle (and especially of matters pertaining to the end of the world, which seems primarily to be meant by these things).
It is well to note here that many of Paul's writings were now in circulationand so well-known that Peter assumes his readers are acquainted with them. As was mentioned in the Introduction, this fact, of necessity, points to a relatively late date for this epistle. Notice, also, that Peter believed Paul's writings to be inspired, for he placed them alongside the other scriptures. Therefore to deny that Paul's writings deserve to be classified as scripture, is also to impeach the truthfulness and inspiration of Peter.

wherein are some things hard to be understood

That is, some of the subjects and statements treated in Paul's epistles. Any careful student of his writings will not find it hard to admit this truth! But notice that Peter does not say, for this reason cast them aside and do not study them! The book of Revelation certainly is not easily understood, but the Holy Spirit says, Blessed is he that readeth, and they that hear the words of the prophecy, and keep the things which are written therein. (Revelation 1:3).

which the ignorant and unstedfast wrest, as they do also the other scriptures, unto their own destruction

The word rendered wrest (strebloo) is akin to streble, a windlass, wrench, instrument of torture, rack. Thus the word properly means to distort (or stretch) the limbs on a rack. Used here metaphorically, the reference is to one who wrests or tortures language (Thayer), i.e., the language and words of scripture.

And who does this? The ignorant and unstedfastoften members of the same class. If they would admit their ignorance, seek out the truth, and abide by it, all would be well for them. But persisting in such ways, their only end is destruction. See notes, 2 Peter 2:1. (On destruction, apoleia, see under 2 Peter 3:6, perished.)