That is no more a valid argument for your unScriptural errors then it is for the Mormons for theirs, and in both cases the veracity of their respective claims rests upon the the novel and unScriptural premise of ensured perpetual ecclesiastical veracity.
Where “in Scripture” is found the chapter and verse that informs you that only “in Scripture” will be found all the many other signs also that did Jesus in the sight of his disciples, “which are not written in this book”?!
Nowhere, and which is another fallacious argument, for Scripture being the only supreme and sufficient standard as the only wholly inspired substantive body of Divine Truth does not and need not claim to know everything that can be known, nor does Catholic tradition claim this. But that Scripture provides, in its material and formal aspects, what is necessary for salvation and obedience to God and growth in grace is evident in the light of evidence.
First, the fact is that even in the Garden of Eden, and before the giving of the Law, then the limited revelation (to a limited amount of people) that God provided was necessary for salvation and obedience to God, but as God gives more grace, so was the case after that.
For when God chose to corporately reveal His will to a entire people then He provided the Law, and which was preserved in writing, this being God's chosen means of preservation. (Exodus 17:14;34:27; Deuteronomy 10:4; 27:3; 31:24; Isaiah 30:8; cf. Job 19:23;John 20:31;Revelation 20:12,15 ) And as is abundantly evidenced, as written, Scripture became the transcendent supreme standard for obedience and testing and establishing truth claims as the wholly Divinely inspired and assured, Word of God.
Go read Ps. 119, and find anything like that for tradition. It was not the Truth of God being passed on by oral tradition that preserved the faith and brought national repentance, but was finding the book of the Law and hearing it. (2Kg. 22,23) Sounds like the supreme and sufficient standard to me for God's purposes, even as regards the Law.
And which the Lord rebuked the devil and leaders by, and established His claims by, and opened the minds of the disciple to, (Mt. 4,22,; Lk. 24:44,45) and which oral preaching was subject to testing by, (Acts 17:11)
To which body additional, complementary writings were added, in conflation with what was already established, which, as with men of God, was discerned as being of God by the people, essentially die to their unique Divine qualities and attestation.
At every stage God provided what was necessary, though He can give more grace, resulting in more blessing as well as accountability.
And i should add here the Scripture alone being the supreme wholly inspired standard for faith and obedience does not wholly consist in formal teaching, like reading Acts 10:36-43 and being able to be born again, but sufficiency also pertains to what Scripture materially provides for, from reason and discernment, to the magisterial office of the church.
The problem is, both the devil and the flesh try to imitate the word of God, and this the question, how is one to ascertain what it is of God? The Catholic response is that only the historical magisterial stewards of Divine revelation can provide this, and dissent from which being rebellion against God.
However, in the 1st. century a prophet began preaching in the desert, followed by another itinerant, non-ordained Preacher, both of whom were rejected by those who sat in the seat of Moses, who, like Caths do, challenged the authority of Him. His response was to demand an answer as to where the first preacher obtained his authority to baptize. The lied and would not say it was of men, not God (or else have to repent) for they feared the people who discerned that this man was a prophet indeed, even though the historical magisterial office rejected him. (Mk. 11:27-33)
Applying all this to our situation today, a evangelical preacher can enjoined obedience to the oral preaching of the word of God, but which presupposed that it is according to Scripture, and its veracity is conditional upon that. But he does not claim to provide new public revelation or to speak as wholly inspired of God, as apostles could.
In contrast, we have Rome which presumed to claim that whenever she speaks infallibility then it is the assured word of God, to which implicit obedience of faith is mandated, as if it were Scripture, even though Divine inspiration as with Scripture is not claimed.
And since Rome has presumed to infallibly declare she is and will be perpetually infallible whenever she speaks in accordance with her infallibly defined (scope and subject-based) formula, then this renders her declaration that she is infallible, to be infallible, as well as all else she accordingly declares. Infallible circularity.
Rome can make belief a binding requirement even in an extraScriptural event which lacks even early historical testimony. , and was opposed by RC scholars themselves as being apostolic tradition. And the veracity of which is based upon the novel premise of ensured perpetual magisterial infallibility.
However, this is not Scriptural, and is contrary to how the church began, and to how God preserved Truth and faith. Thus the church began upon the secondary foundation of rebels - apostles and prophets - against those who presumed too much of themselves,
JOHN 20:31