Peter Warns of Paul's Writings
Peter writes in his second epistle, Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness. Wherefore, beloved, seeing that ye look for such things, be diligent that ye may be found of him in peace, without spot, and blameless. And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; as also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness. But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen.2nd Peter 3:13-18
Here, Peter mentions Paul, the only writer in the New Testament to do so, besides Luke. Peter says, the longsuffering of our Lord is salvation; even as our beloved brother Paul… hath written to you.2nd Peter 3:15 So Peter agrees with one thing that Paul has written. (Peter was possibly referencing Romans 2:4, or maybe even 1st Timothy 1:16.)
Then Peter thinks to speak about Paul's epistles, saying, in all [Paul's] epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness.2nd Peter 3:16, 17 "Wicked" here is literally ἄθεσμος, athesmos, Strong's G113, or "lawless."
Peter is not saying that Paul, nor his writings, are correct. He is saying that Paul has one thing correct, that "the longsuffering of our Lord is salvation." Of the others, he warns that unlearned people try to establish those scriptures, to their own destruction!
Last, let us make clear what Peter is saying when he writes in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. He is saying that the unlearned and unstable are trying to wrestle the other scriptures into their own interpretations, rather than listen to what they say. Nowadays, we would say that these people "twist" the scriptures.
Value of this Evidence
This should be indefensible evidence against Paul as an apostle. Peter is saying to his readers to beware Paul's writings! However, the modern day interpretation of these verses ignores this warning, and only focus on two parts of this passage: that Peter thought Paul's writings were confusing; and that Peter thought Paul was a brother.