You started out with the statement that the Bible alone is not the arbiter of truth. With this I agree. Jesus said that the holy Spirit would lead us into all truth. The holy Spirit inspired the scriptures. So the holy Spirit would never contradict what He has already revealed through the prophets and apostles. Then you say...
This is a classic strawman. No-one is claiming that they are the arbiter of truth.
KJV John 8:31-32
31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
32 And ye shall know the truth, and the truth shall make you free.
KJV John 14:6, 15-17, 23-26
6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
15 If ye love me, keep my commandments.
16 And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever;
17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
23 Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.
24 He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father's which sent me.
25 These things have I spoken unto you, being yet present with you.
26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
The following prayer of Paul as written to the Ephesians, was written to Gentile converts, not to an elite self spiked hierarchy in Rome... Take note of what he is asking for on behalf of these lay believers in Christ...
KJV Ephesians 1:15-23
15 Wherefore I also, after I heard of your faith in the Lord Jesus, and love unto all the saints,
16 Cease not to give thanks for you, making mention of you in my prayers;
17 That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him:
18 The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints,
19 And what is the exceeding greatness of his power to us-ward who believe, according to the working of his mighty power,
20 Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places,
21 Far above all principality, and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come:
22 And hath put all things under his feet, and gave him to be the head over all things to the church,
23 Which is his body, the fulness of him that filleth all in all.
This prayer as presented in God's word becomes a promise... Not to a magisterium of "suitably" qualified individuals in an institution... But to you and me. Think about what is promised here. The work of the holy Spirit in man reveals God. Reveals His will. And uses God's word as the vehicle for that will when it comes to doctrine and Truth.
you cannot know the truth or the scriptures or have salvation with Christ and his church!
they are inseparable covenant of God!