There are several levels on which this may be challenged; that is to say, the claim that the early apostles actually believed that their writings, in any way, rose to the same level of the Old Testament Scriptures.
(a) The earliest church fathers and in particular, the first acknowledged church historisan, Eusebius, holds this epistle, 2 peter, to be a forgery.
(b) When paul asked Timothy to “Bring the cloak that I left with Carpus at Troas when you come–and the books, especially the parchments.”, did he honestly believe that this was Holy Writ?
(c) Technically, it cannot be denied, that there are two words in play here; in verse 16, “Peter” uses the word “letters” (Epistles) and the word “Scriptures” (writings). Much is made by Christian teachers, that the word “other” means that “Peter” was comparing Paul’s “letters” with the “scriptures”.
(d) There is NO record of the word “scriptures” meaning Holy Writ before the early 1300’s. So one is forced to ask, (1) What did scriptures mean before that, and (2) what changed that?
The greek word is “graphe” from which we get the english word “graphite” which is what we find in the center of a lead pencil. “Graphe” means to write, or written or writings. When the Bible was Latinized, the translators used the Latin word “scriptura” meaning exactly the same thing – “writings. The Latin word, hase made it into English as the words, script, and manuscript, etc. Hence we have in English today, derivations from both the Greek AND the Latin which are very clear in every thing EXCEPT theology. So what changed this?
Around the first time we have any evidence of “scripture” being made to describe Holy Writ, is when the Roman Catholic Church found heretical and excommunicated one, William of Occam. This was the same man after whom was named the term, “Occam’s Razor“. It is sufficient for this article to state that William of Occam prized logical thought over mindlessly following tradition. For this, he was declared to be a heretic and excommunicated from the Roman Catholic Church. Remember that the word “heresy” in the original Greek means to think for one’s self. This was the same church that declared Galileo a heretic for teaching that the earth revolved around the sun instead of the opposite, they would burn him at the stake.

The fact that we may suppose the word “Scripture” to include any New Testament writer, is mute testament to the fact of how comprehensively we have been brainwashed by the selective ignorance of modern christian theology.

Only by way of serious linguistic manipulation, and/or ignorance, can 2 Peter 3:16 be made to mean that any New Testament scripts or graphe, or writings equate to what is called in today’s Christianity, scripture!