The authorship of 2Peter is disputed; the vast majority of scholars today do not believe the Apostle Peter wrote 2Peter. Even if Peter did write 2Peter, he calls Paul's writings to be "graphe" which is literally translated "writings". (The third, and least important, category of Scripture is the "Writings" section - the most important two includes the Torah/Law and the Neviim/Prophets. The Writings are considered edifying, but not necessarily inspired - much like any good sermon today. (Alas, the different levels of authority in Scripture is something not known to most of mainstream Christianity today. Please see my previous post here , if you'd like, for more details on the three divisions of Scripture.)
Lastly - even if we accept the book of 2Pet, 2Pet 3:16 is misunderstood by many Christians today, as they forget the advice of the very next verse. This verse actually warns against those who misunderstand Paul's writings, as they may be led away by the error of the athesmon/lawless (2Pet 3:17)! The author of 2Pet is warning readers that, if they read Paul, they should not fall in their walk by adopting the erroneous idea that the Torah/Law was done away with (cf Mt 5:17-19)! Also note that 2Pet never called Paul an apostle.
If the street preacher, from my previous example, wrote an epistle today stating that the apostles James, Peter, and John came down from heaven to him and approved of his message, will you accept his epistle as first-hand evidence from the apostles, or would you consider it to be second-hand and hearsay information?
If the street preacher's friend (let's call him Luke) wrote a history book today, stating that Peter and James sent some friends to accompany the street preacher, will you accept this testimony as first-hand witness from Peter and James, or would you also consider it second-hand & hearsay?
Neither the street preacher's epistle nor his friend's history book would fulfill YHWH's requirements which demands
first-hand witnesses. Secondly, if the street preacher's epistle claims a message contrary to YHWH's previously established Scripture (the Torah, Prophets, and Writings), then we know for sure that the epistle is NOT from YHWH (Deu 13:1-5, Deu 18:18-22, Isa 8:20).
In the same way, Galatians is the word of Paul of Tarsus
only. It is not the first-hand testimony of Peter, James, or John. Acts is the second-hand information gathered together by Luke. It also, is not the first-hand testimony of Peter or James or anyone else.
It doesn't matter if the street preacher is Paul of NY on the streets of New York City today (1,980 years after the resurrection of Messiah), or "Paul of Tarsus" on the streets of Galatia, 10 years after Messiah's resurrection. The standard and rules are still the same.