Authority being established (versus inherently having) based upon evidential warrant - or the authority of evidence or upon a prior established authority is sound, and thus even the veracity of the oral preaching of apostles was subject to testing by Scripture (Acts 17:11) as the prior established authority of OT Scripture. Which in essence was established upon its unique heavenly qualities and attestation.
And which provided the doctrinal and prophetic epistemological foundation for the gospel and thus the church. (Romans 1:1,2; 16:26) Therefore it was Scripture that the Lord Himself invoked, from defeating the devil (Mt. 4) to correcting Jewish leaders (Mt. 22) to substantiating His messiahship and ministry ("in all the Scriptures") and which He opened the minds of the disciples to them, who did the same. (Luke 24:27.44,45; Acts 17:2; 1828, etc.)
Thus while Christ had inherent authority, His claims were established upon the prior established authority of Scripture as well as the miraculous which principal of attestation that Scripture establishes. (1 Kings 17:24)
However,
it is a very short step from “evidential” authority to the authority of the church. In a switch from mere evidences, to churchly authority, Scripture is still dependent on something outside of itself so that one’s epistemology remains intact, but it is now “baptized” by the church.
While the author attempts to make the authority of Scripture dependent upon the authority of the church, meaning her self-proclaimed infallible decree of what Scripture consists of, yet the authority of the church latter is dependent upon the former. And which was did not require an infallible magisterium for the establishment of an authoritative body of wholly inspired writings, by which Christ substantiated His mission ("in all the Scriptures").
And as before and as with men of God, more writings became recognized and established as being of God, but which do not support the presumed supreme infallible authority of the Catholic church, Western or Easter Orthodox. For distinctive Catholic teachings are not manifest in the only wholly inspired substantive authoritative record of what the NT church believed (which is Scripture, in particular Acts through Revelation, which best shows how the NT church understood the gospels).
In addition, in Catholic theology it is taught that that one cannot know writings are of God except by faith in her which tells you, and thus "when we appeal to the Scriptures for proof of the Church's infallible authority we appeal to them merely as reliable historical sources, and abstract altogether from their inspiration." (Catholic Encyclopedia > Infallibility) Whereby it is supposed that while one cannot discern the Bible as being of God, yet it is supposed that one can discern the Catholic church as being of God. And thus one becomes dependent upon her for the meaning of it, which excludes Scripture from contradicting her, which it does.
And which Catholic premise also means that her own authority is not established upon appeal to the authority of Scripture, but upon herself. For 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, which renders her declaration that she is infallible, to be infallible, as well as all else she accordingly declares.