Excuse me, how do we know the Roman magisterium is backed by the Triune God?
The Bible is self-authenticating because of prophecy which validates it. The same is not true of the BOM and the Koran.
Since 1987 a card-carrying* Baptist Fundamentalist, I do not say the Bible is self-authenticating. The prophecies and the miracles authenticate it if they are true. Conviction by the Holy Spirit also authenticates it in the same way that you would authenticate a witness testimony by looking into the eyes of the witness and knowing that he speaks the truth. It is better to think that the Bible authenticates Jesus Christ. But I would prefer not to get into strivings of words.
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*Yes I have an actual card in my wallet. It is a membership card signed by Dr. Rod Bell and issued by the Fundamental Baptist Fellowship, dated 1987. Now renamed the Foundations Baptist Fellowship International, (I am late paying my dues this year) we had a Southwide Region meeting on zoom last month with Pastor Tony Facenda as moderator.
Well it cannot be because the authoritative Scriptures show this, for besides the fact that they do not, "the believer cannot believe in the Bible nor find in it the object of his faith until he has previously made an act of faith in the intermediary authorities..." (Catholic Encyclopedia>Tradition and Living Magisterium) People cannot discover the contents of revelation by their unaided powers of reason and observation. They have to be told by people who have received in from on high. - Cardinal Avery Dulles, SJ, "Magisterium: Teacher and Guardian of the Faith, p. 72
Therefore, since one needs faith in Catholicism and instruction by it in order to know what writings are of God, then (to avoid circular reasoning),
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)
By which appeal the candidate for conversion is sppsd to be able to ascertain that the RCC or the EOC is of God, even though it is impossible for said candidate to know that which God-inspired writings are of God.
Thus apart from submitting to Rome (or the EO), you cannot know for sure if you are correct since you are fallible. Therefore while you may think the RCC cannot be the NT of Scripture, yet you cannot be sure. However, if you make a fallible decision to submit to her, then you can know that she what she says she is, because 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 that renders her declaration that she is infallible, to be infallible, as well as all else she accordingly declares.
You asked. .