[Paul] writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.
(2 Peter 3.16)
Discussion on the doctrine of scripture always follow with the reference to 2 Peter 3.16 where Peter referred to Paul's letters as part of scripture. Christians who want to defend those Paul's letters in the New Testament (Romans, 1-2 Corinthians, etc) as scriptures always quote this verse.
But there is one mistake in the argument.
We don't know which letters was Peter referring to when he talked about Paul's letters. Paul wrote more than what we have in the New Testament (1 Corinthians 5.9). May be Paul did wrote a few letters which the other apostles deem as scripture but we don't know which one, even if they are in the New Testament.
Now we have a dilemma.
1) To claim God's authority for all of Paul's letters in the New Testament risks elevating letters that are not authorized (1 Corinthians 7.25) as scripture. A form of idolatry.
2) To claim all of Paul's letters in the New Testament not as scripture risks disregarding those which are scripture. A form of desecration.
So in the end we are left with another ambiguity.