We can start off with the belief, and I don't want to say *assumption*, that if it's in Scripture, it's true because it is TRUTH.
On the other hand, being not found in Scripture does not by default mean that it's not true.
However, if it's not found in Scripture, it can't be presumed to be true to the level of Truth as what IS found in Scripture and so must be compared to Scripture and supported by Scripture to give a valid reason for accepting it as truth.
If it can be shown to be contradicted by Scripture, then it can be demonstrated as false.
AHhhhh....
We protestants do not assume that the majority vote of a large group of men as capable of discerning doctrinal TRUTH that is NOT found in Scripture.
Of that I have no doubt. It's a common theme with all non Christian religions and aitheism.
Well then you have just joined the ranks of Mormons and cults, and you must expect us to believe the word of who, that there was another transcendent supreme Infallible Authority throughout the history of the people. Was the Jewish magisterium an Infallible Authority?
And why should be believe that Rome is that supreme Infallible Authority if it is not in Scripture?
However, the context clearly was Scripture, as you stated that you looked through a handful of Scripture texts that testify to Scripture being the transcendent supreme standard for obedience and testing and establishing truth claims as the wholly Divinely inspired and assured, Word of God, but did not seen anything that establishes the sole infallible authority of Scripture.
Yet as it uniquely exalts the written word as "perfect, converting the soul" and making wise the simple and "more to be desired are they than gold," etc. (Psalms 19:7,10)
and is what the Lord rebuked the devil by (Mt. 4)
and is what the Lord established His mission by to the disciples and opened up their mind to, (Lk. 24:44,45)
and which transcendent material source alone is stated to be wholly inspired by God,
and which amorphous oral tradition must conform to,
then it alone is the supreme transcendent Infallible Authority (which by nature must have definite form), which even the claimed infallible discerners of amorphous oral tradition (all the words of which are not wholly inspired) must establish their legitimacy by, but cannot.
Nor establish that the truth of any of her claimed oral traditions were not subsequently written, or that Scripture does not formally and materially together provide what is needed for salvation and growth toward perfection (which includes oral preaching of Scriptural truths, and the Spirit's guidance, subject to Scripture).