I have been listening to debates lately between Protestants and Roman Catholics on the issue of sola scriptura. For a debate question such as "Sola Scriptura or the Sufficiency of Scripture is taught by Scripture", the Protestants will of course take the affirmative side. Now of course, the Protestants do not mean to say that the phrase "Scripture is Sufficient" exactly can be found in the Bible, but is this principle taught just like the Trinity? The word Trinity is not found in the Bible but does the Bible teach such concept about God? Simply because the word is not found in the Bible, it does not mean that the Bible does not teach it.
Now for the RC apologist, what must he do to effectively rebut the affirmative? The Protestants believe that sola scriptura is vouched by 2 Tim 3:14-17. The RCs have to do more than say "I do not accept that". Also the exegesis that limits the application of this Scripture here to OT does not wash, because you will have to deny that the NT is not Scripture and I do not think you would like to go there as it will lead to a can of issues.
Think of this way, what is the Protestant position really saying? They are saying "The Bible teaches that there is only one source for a Christian's faith and conduct - it is the Bible". This is what is being said here.
This is similar to me saying - "I can prove to you that there is only One God". For you do take the negative on this you would have to prove, either of two things. You prove that there is no such God or you prove that there is at least another God and this one is not identical to the other.
For this case, the former is invariably taken as the method used by Atheist because by definition, God is supreme being and therefore unique, that is why we do not hear the latter position taken as a method (that is prove at least 2 God etc.) of proof.
From the debate I hear, RC apologists normally use 2 Th2:15 where Paul says "hold to the traditions that you were taught by us, either by our spoken word or by our letter" to show that sola scriptura is denied. Their point ,from what I understand, is the "spoken word" along with the "letter (Scripture)", thus it is not Scripture alone it has to be the oral words. Notice too that it says "either", that is this tradition is found in either spoken word or letter.
To be successful they have to show that "the spoken word" is NOT the same or is NOT identical with the "letter". That is, the contents of the two must not be identical with each other, for if they were, then they have not denied the position of the Protestant successfully. Thus they have to do a bit more work.
More later, God bless you in your thinking