The Scriptures referred to in these passages is in fact the OT, which Jesus used to show that He is the Messiah.
How could Jesus have died and rose according to the NT Scriptures that had yet to be written?
Remember, when the letter to the Corinthians was written it was not considered scripture.
I know. It was OT Scriptures Paul was referring to.
Act 20:27 For I have not shunned to declare unto you all the counsel of God.
Paul did not teach only what was contained in his epistles...Paul taught the OT as pertaining to Jesus...Paul taught the Gospels as well...
Remember, when the letter to the Corinthians was written it was not considered scripture.
There's no reason to believe it wasn't...
2Th 2:15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
IF you'll look at this verse that you are very familiar with, closely, you'll see that 'traditions which you have been taught' ARE the written epistles...The oral traditions ARE scripture...
Unless of course, you switch the or to an and...Which you guys do...