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Actually, the early copies of Peter’s epistle do not indicate “other scriptures,” but simply “scriptures,” so the “other” was obviously added centuries after peter wrote his letter.
No, the apostles were not arrogant enough to assume their letters were “scripture,” Tanakh was “Holy” (set apart) to them.
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Nonsense; P72, the oldest manuscript of the work we have clearly contains the reference. In context, also, Peter is clearly referring to Paul’s letters as scripture. Paul writes claiming Apostolic authority and obviously believes his letter are divine authority for the lives of those he addressed them to. There were intended to be read with the OT scriptures in the earliest church and the Apostles knew this was being done. None objected to the churches treating their epistles as scripture. I have no doubt the Apostles considered their letters to be scripture as much as the recipients did.