Paul was of course referring to the much studied and proclaimed OT Scripture which had long been in existence, which Timothy had been raised with.
But we should also be aware of the
"three as endorsement" Peter gives of Paul's writings.
It is not that Paul claimed to be writing Scripture, but that what he wrote was acknowledged as Scripture, at first by Peter, and later by those God guided in collating the canon of the NT.
2Pe 3:14-16 Therefore, beloved, looking forward to these things, be diligent to be found by Him in peace, without spot and blameless; (15) and consider that the longsuffering of our Lord is salvation—
- as also our beloved brother Paul, according to the wisdom given to him, has written to you, (16)
- as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction,
- as they do also the rest of the Scriptures.