What is the Bible's internal proof that it is the Word of God?

Yes, by all means lets rewrite God's word! The Saints of the Old Testament obtained approval by means of (not faith, but) proof.
Thomas required evidence, but Jesus said we will believe without such evidence.

The people who put together the canon of books we call the Bible used various tests, such as to recognize God's word. Such as the initial audience accepted the writings as God's word. One test is does the text change lives, is it dynamic. If a person simply hears God's word, then wonders whether to believe it, the likelihood of it changing their lives is remote.

 

WCF/LCF:

The authority of the holy Scripture, for which it ought to be believed and obeyed, dependeth not upon the testimony of any man or church, but wholly upon God (who is truth itself), the Author thereof; and therefore it is to be received, because it is the Word of God.

We may be moved and induced by the testimony of the Church to an high and reverent esteem of the holy Scripture; and the heavenliness of the matter, the efficacy of the doctrine, the majesty of the style, the consent of all the parts, the scope of the whole (which is to give all glory to God), the full discovery it makes of the only way of man’s salvation, the many other incomparable excellencies, and the entire perfection thereof, are arguments whereby it doth abundantly evidence itself to be the Word of God; yet, notwithstanding, our full persuasion and assurance of the infallible truth and divine authority thereof, is from the inward work of the Holy Spirit, bearing witness by and with the Word in our hearts.

 

Ziggy has approached the question from the proper direction.

I think Guido is asking: WHAT PROVIDES THE AUTHORITY OF SCRIPTURE?

There is internal evidence (evidence found within Scripture, examples, its own testimony [Psalm 119!]…it’s trustworthiness, it’s internal cohesion among various authors, in different cultures, and over a thousand years.

…and external evidence (evidences found outside of Scripture, examples include history, archeology, testimonies,

The Bible is incomparable with religious writings in other belief systems.

… and of utmost importance, our simple faith that God loved mankind and provided a means of salvation in his Son.

Rob

 

As a writer, the internal evidence of Divine activity upon the words that have been preserved for us (including the process by which oral tradition was turned into writing and texts put together) is indicated by the cohesiveness of the message of the books -- written in at least three languages; from an unknown, but significant number of authors; through at least four to five massive cultural shifts; and from people with wildly diverse backgrounds and influences.

The external evidence of Divine activity upon the words preserved for us includes millennia of individual and corporate experiences with the story of the text that corresponds to share experience with God. That does not mean that everyone agrees on everything, but that persons and communities find the experience of engaging with the scriptures transformative.

My personal evidence of Divine activity upon the words preserved for us includes my own objective study of the Bible as an honest agnostic -- I realized that I could not fairly reject something that I had not read for myself and have any integrity as a person with an opinion about the Bible. I real the Bible with an open and critical (in the best sense) mind and discovered by the end of it that it was not what I had been taught it was in the church in which I was raised (Baptist). My childhood teachers had created a canon of morality tales with mostly faithful biblical characters that did the right thing and were rewarded by God. However, life is not so simple. I realized that I had previously had a childish faith that could not support the realities of being a young adult. I decided to reread section that had could my attention, which led from one thing to another. Within a month or two I had ended up reading the Bible through for a second time. By then, I was seeing how it all fit together. I still had a lot of questions and unresolved issues, but I felt for the first time what the Christian message was really about. I was very proud of myself until I realized that, while I am intelligent, I am not smart enough to put together the understanding of the scriptures that I had already developed. I realized that in all of those late nights of study, insights would appear to me unbidden, which would turn my attention to other scripture, opening up new understanding. I realized that I was being TAUGHT in my study from Someone beyond myself. That's when I began to pray as an adult, and my life has never been the same.

 

We have a two fold evidence. The Apostle John in his letter calls upon them.

1 John 5:9, ". . . If we receive the witness of men, the witness of God is greater: . . .". Of course John has specifics in mind in his letter. Not all of Holy Scriptures.

Epistemology is about how we known. Most of what we know comes from others.

 

An otherwise obscure reference in the Song of Songs 8:6, ". . . Set me as a seal upon thine heart, as a seal upon thine arm: for love is strong as death; jealousy is cruel as | Sheol: | the coals thereof are coals of fire, which hath a most vehement flame | of the LORD. | . . ." [Deuteronomy 32:22. ]

 

And I think that one of the Bible Proofs of itself and the TOTALLY DEPRAIVED NATURE OF ALL MANKIND AFFECTED BY THE SIN OF ADAM is the DENIAL of the Adamic sin Imputed upon all his posterity.

 

We have a "more sure Word of prophecy", i.e., more sure than prophecy, which was impeccable.