" If we are not to rely on the mediation of a divinely ordained church possessing apostolic authority then the Holy Spirit must be failing one of us (or perhaps both)."
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Clearly this is where you lost your way. It is a difficult journey when you've been raised within a system that tells you not to think, or question things for yourself, but to only believe what they tell you.
BTW, there are no Apostles living today.
You are obviously unfamiliar with the method of teaching Catholic theology. The Church has always shunned pure fideism and insisted on the ability to reason and come to sound conclusions. If you were to join with a gathering of Catholic theologians you would find that the debates would be quite intense. The modern university is a product of this legacy.
I would also maintain that you are no less beholden to Protestant tradition than the Catholic is to his.
As for the Apostles, their office does continue to this day:
For it is written in the book of Psalms: Let their habitation become desolate, and let there be none to dwell therein. And his bishopric let another take. Wherefore of these men who have companied with us all the time that the Lord Jesus came in and went out among us, beginning from the baptism of John, until the day wherein he was made a witness with us of his resurrection. And they appointed two, Joseph, called Barsabas, who was surnamed Justus, and Matthias. And praying, they said: Thou, Lord, who knowest the hearts of all men, shew whether of these two thou hast chosen to take the place of this ministry and apostleship, from which Judas hath by transgression fallen, that he might go to his own place. And they gave them lots, and the lot fell upon Matthias, and he was numbered with the eleven apostles.We can also see this in the work of St. Paul in appointing bishops in the churches he established.
(Acts 1:20-26)