Canon

There is a comprehensive encyclopedia article on why there is only a limited selection of books and other works considered to be part of the original divinely inspired Scriptures that make up what we know as the Bible. It goes fairly in-depth on quite a number of arguments, to expand on and explain the opening statement:

The mere writing of a religious book, its preservation for hundreds of years, and its esteem by millions do not prove it is of divine origin or canonical. It must bear credentials of Divine Authorship demonstrating that it was inspired by God. The apostle Peter states: “Prophecy was at no time brought by man’s will, but men spoke from God as they were borne along by holy spirit.” (2Pe 1:21) An examination of the Bible canon shows that its contents measure up to this criterion in every respect.

The online version of the article is publicly available here https://wol.jw.org/en/wol/d/r1/lp-e/1200000880, and the paper print may be available locally at no cost (as in having a price of $0).

Apocrypha

There is also an article in the same encyclopedia about the reasons why the different apocrypha, or deuterocanonical books, were not included.

Why is the NT considered Scripture?

That is exactly what Apostle John a disciple of Jesus sought to address to those who questioned the accounts of the disciples and those who had first experience with Jesus.

“We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error.” ‭‭1 John‬ ‭4:6‬ ‭

Who is the us?

John is referring to those who were disciples of Jesus

“That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life— the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us— that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ. This is the message we have heard from him and proclaim to you” ‭‭1 John‬ ‭1:1-3, 5‬

Then the question arises what about Apostle/Emissary Paul’s writings?

His writings were prophesied in the writings of the Essenes in the DSS that a Benjamite would come on the scene and do exactly what Paul did.

But Biblically after spending time in Arabia Paul went back to Jerusalem and consulted with the Apostles who were first hand witnesses of Jesus and confirmed that what he had received from the Holy Spirit aligned with what Jesus told the disciples

“But when he who had set me apart before I was born, and who called me by his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles, I did not immediately consult with anyone; nor did I go up to Jerusalem to those who were apostles before me, but I went away into Arabia, and returned again to Damascus. Then after three years I went up to Jerusalem to visit Cephas (Peter) and remained with him fifteen days. But I saw none of the other apostles except James the Lord's brother. (In what I am writing to you, before God, I do not lie!)” ‭‭Galatians‬ ‭1:15-20‬ ‭

And Peter (Cephas) agrees with Paul and acknowledges his writings

“And count the patience of our Lord as salvation, just as our beloved brother Paul also wrote to you according to the wisdom given him, as he does in all his letters when he speaks in them of these matters. There are some things in them that are hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures.” ‭‭2 Peter‬ ‭3:15-16‬ ‭

Paul acknowledges that he did not receive his gospel from men but directly from Jesus and only afterwards did he consult with Peter and James

“For I would have you know, brothers, that the gospel that was preached by me is not man's gospel. For I did not receive it from any man, nor was I taught it, but I received it through a revelation of Jesus Christ.” ‭‭Galatians‬ ‭1:11-12‬ ‭

So the NT or the new covenant writings are Scripture because Apostle John and Peter witness to them as being accurate and in line with the words of Jesus.

Why is the OT Scripture? That has a long answer and also a short answer but I’ll only address the short answer

“For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.” ‭‭2 Peter‬ ‭1:21‬

And God did nothing without first telling His prophets and all the patriarchs from Adam, Seth and so forth were prophets and also had a book that they passed onto their children and copies of these survived in the DSS

“"For the Lord God does nothing without revealing his secret to his servants the prophets.” ‭‭Amos‬ ‭3:7‬ ‭

Also the Bible has a list of books that it recommends as a reading list which are not in the Bible but endorsed by the Bible

And Jesus and the Apostles Jude and Peter quote from writings of the patriarchs which are not in the Bible but endorsed also therefore also Scriptures.

Furthermore Scripture explains Scripture. Jesus demonstrates this with the two on the road to Emmaus.

The OT is the NT concealed and the NT is the OT revealed.

When the Bareans listened to Paul they went home and checked the OT to see if what Paul was saying about the new covenant (NT) was consistent with Scripture (OT).

The Greek word ‘rhema’ refers to the actual spoken/written words of God (Hebrews 6:5). When Jesus was being tempted by Satan, He answered,

“Man does not live by bread alone, but by every word [rhema] that proceeds from the mouth of God” (Matthew 4:4).

Ephesians 6:17 says “take the helmet of salvation and the sword of the Spirit, which is the word [rhema] of God.”

‘Logos’ is another Greek word that is applied to the written message of God (John 17:17; 1 Timothy 4:5; Revelation 1:2; Colossians 1:25). Hebrews 4:12 says,

“The word of God is alive and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart.”

There is also a link between ‘logos’ as applied to the written word of the Bible and to Jesus in that He is the subject of the written Word:

“You study the Scriptures diligently because you think that in them you have eternal life. These are the very Scriptures that testify about me” (John 5:39).

Indeed, in John 1:1-3 and 14, the pre-mortal Jesus is known as the Word (logos), who was with God in the beginning, and who is God.

God communicates with us through the prophets, through His written word (the Bible), through the spoken word (the gospel of the good news about Jesus) and through Christ Jesus himself.

In the past God spoke to our forefathers through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son” (Hebrews 1:1-2).

And where do we find what God has spoken? Why, in the written Word of God, the account of the prophets (in the Old Testament) and the account of Christ Jesus (in the New Testament). That is why the four Gospel accounts of the life, death and resurrection of Jesus, along with the other letters about what happened after the Holy Spirit kick-started Christianity after Pentecost are considered to be Scripture. They are all about the person of Christ Jesus (the Logos or the Word of God) and the glorious good news of the Gospel, which is also about Christ Jesus.

Karl Barth believed that Jesus is the Word of God, but that the written Bible is NOT the Word of God. He believed that the Bible becomes the Word of God when preached by the church. See https://postbarthian.com/2018/08/20/karl-barths-doctrine-of-the-word-of-god-in-threefold-form/

Barth then argues the Bible becomes the Word of God when it is proclaimed by the living human voice of the Church, and only in that event may the Bible be rightly called the Word of God. Barth explained "the Bible, then, becomes God’s Word in this event, and in the statement that the Bible is God’s Word the little word 'is' refers to its being in this becoming. It does not become God’s Word because we accord it faith but in the fact that it becomes revelation to us."

On the other hand, Fundamentalists, Evangelicals, and even Catholic traditions going back to Augustine subscribe to various theories of inerrancy and canon formation. This article has some intersting points to make about where the ideas of inerrancy come from:

https://www.thegospelcoalition.org/article/did-fundamentalists-invent-inerrancy/

Many people rely on their view as handed down to them by their church statement of faith, denominational position, or an ecumenical creed. One such creed is the Chicago Statement on Biblical Inerrancy:

http://www.romans45.org/creeds/chicago.htm

Here is the summary statement of the much longer document:

I. SUMMARY STATEMENT

1. God, who is Himself Truth and speaks truth only, has inspired Holy Scripture in order thereby to reveal Himself to lost mankind through Jesus Christ as Creator and Lord, Redeemer and Judge. Holy Scripture is God's witness to Himself.

2. Holy Scripture, being God's own Word, written by men prepared and superintended by His Spirit, is of infallible divine authority in all matters upon which it touches: It is to be believed, as God's instruction, in all that it affirms; obeyed, as God's command, in all that it requires; embraced, as God's pledge, in all that it promises.

3. The Holy Spirit, Scripture's divine Author, both authenticates it to us by His inward witness and opens our minds to understand its meaning.

4. Being wholly and verbally God-given, Scripture is without error or fault in all its teaching, no less in what it states about God's acts in creation, about the events of world history, and about its own literary origins under God, than in its witness to God's saving grace in individual lives.

5. The authority of Scripture is inescapably impaired if this total divine inerrancy is in any way limited of disregarded, or made relative to a view of truth contrary to the Bible's own; and such lapses bring serious loss to both the individual and the Church.

Some people rely on certain Bible passages to justify their beliefs about what the Bible is. Thessalonians equates the Scriptures with God's universal, authoritative, and powerful truth:

13 And we also thank God continually because, when you received the word of God, which you heard from us, you accepted it not as a human word, but as it actually is, the word of God, which is indeed at work in you who believe. (1 Thessalonians 2:13)

Peter's words are used to establish that Paul's letters are part of Scripture:

16 He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction. (2 Peter 3:16)

Similar bootstrapping techniques are used to tie in the other New Testament letters and books. As for the supernatural power and efficacy of the Word of God:

12 For the word of God is alive and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart. (Hebrews 4:12)

On the other end of the spectrum, you have people whose justification for the Bible being the Word of God is because it is self-authenticating. The Holy Spirit applies the words of the Bible to a person's life and they are transformed. Their personal experience gives witness to their belief. I began as person who accepted it as told to me by people I trusted (creedal) and ended up as a person who experienced the power of the Bible to change my life. For example, my Bible Study leader in college asked us all to memorize Galatians 2:20-21:

20 I have been crucified with Christ and I no longer live, but Christ lives in me. The life I now live in the body, I live by faith in the Son of God, who loved me and gave himself for me. 21 I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing!”

A month later, my fear of death went away. In like fashion, I attended a retreat to study Philippians, and Joy entered my heart and overcame a decade of depression. A passage in Isaiah and a Psalm stopped the nightmares I used to have all the time. The Sermon on the Mount helped me to not feel poor and incapable of helping others. I could go on and on. Ultimately, it is the believer's personal experience of deliverance and growth that proves that the Bible is the Word of God.