There are 66 books in the Bible. Taken together, these are known as the canon1 of divine revelation. They were written over a period of around two thousand years by about forty different writers. However, just one author inspired them—God. But how can we be sure that these 66 books are the only inspired Scriptures, no more or less?

The Old Testament

The ancient Hebrews had a clearly defined body of Scriptures. The Word of God was recognized from the very beginning, and there was no doubt which books belonged. The Jews did not arrange their books in the same way as Christians do, however, and some books were combined. But the content was the same. No more or less.

Evidence within the Bible indicates that most of the books of the Old Testament were written at the time of the events they record, and they were accepted as God’s Word at the time the words were given. Historical records outside the Bible reinforce this conclusion.

Jesus verified that the Old Testament writings were always Scripture when He quoted from the Old Testament and claimed, “It is written” (Matthew 4:4, 7, 10) and “Scripture cannot be broken” (John 10:35, NIV). Paul had no doubt that the Jews were “entrusted with the very words [the oracles] of God” (Romans 3:2, NIV). Modern-day theologian Carl F. H. Henry wisely concluded, “The church inherited the Old Testament, and Jesus defended, encouraged and exemplified faithful submission to these writings as an inspired canon.”2

A famous Jewish council met at Jamnia around AD 100. Its purpose was not, as is often erroneously suggested, to decide which books would be included among the sacred writings for the Jews. The purpose was which books were already accepted.3 The council had access to the same Old Testament canon that we have today.

The New Testament

Evidence within the Bible supports the conclusion that most of the New Testament was written by AD 70, while the writings of John came a little later.4 Most of the books name the author, and all the books give other helpful clues, which the book of Acts helps us to date more accurately.

Several statements in the Bible indicate that the New Testament would be written by apostles or approved by them. Peter called Paul’s writings “scripture” (2 Peter 3:16), and Paul said he and other apostles spoke “the word of God” (1 Thessalonians 2:13).

Before the close of the first century, Clement of Rome either quoted from or referred to more than half the New Testament books and called them “scripture.” By AD 180, Irenaeus of Lyons quoted over 1,000 passages from all but a handful of the New Testament books, calling them “holy Scriptures” given by the Holy Spirit. At the same time, Tertullian from North Africa referred to the “New Testament” and expounded on most of it. Origen from Alexandria in AD 240 referred to our 27 books as Scripture, and Athanasius used the same list in AD 367.5 They used no other books in the same way.

It is certain that by the early second century, the four Gospels (and never any others) and the thirteen letters of Paul were accepted by the churches across the Roman Empire without question.

Up to the year AD 180, all our New Testament books (with the single exception of 2 Peter) are found in either direct quotations or allusions in the writings of the leaders of the churches. By this year, a few churches hesitated over James, Jude, 2 and 3 John, 2 Peter, Hebrews, and Revelation; but all the rest were universally accepted. The church leaders never used noncanonical books with the same authority as the New Testament books.

The Muratorian Canon, originally compiled around AD 150, is our earliest documented evidence of a body of books that was identified as the New Testament canon of Scripture. It contains all but four of our New Testament books, though the only surviving copy from the eighth century is in poor condition and is missing parts. No council was ever called to debate the contents of the canon—it was almost universally recognized for what it was, the Word of God.

Why did it take so long for the list of the New Testament canon to show up?

The apostles did not leave us with a neat list of authoritative books.

No scroll could contain all the books, and the process of making books (called codices) did not become popular in the Roman Empire until the fourth century.6

The churches were widely scattered across the Roman Empire and beyond, from Britain to North Africa and east into Persia. Early Christians were still facing persecution and could not easily meet together.

Even though no one church or leader had authority to dictate to the others, it is amazing that close to AD 150 the Muratorian Canon could list all but four of our New Testament books.

Many deceivers wrote false gospels and letters, pretending that they were written by the apostles. But the early church leaders dismissed them as counterfeit and unreliable.7 The false gospels and letters betrayed themselves by their late date of composition—well beyond the time of the apostles—and by their teaching, which clearly conflicted with the canonical books and the accepted doctrines of the churches.

Conclusion
History reinforces our confidence that God gave us His entire Word and preserved it for us, as He said He would. We have precisely the books that God planned from eternity—66 books in all.

Footnotes
1. This comes from the Greek word ????? (kanon) which means “reed.” It came to mean “ruler” or “measuring stick” and later had the idea of a “recognized authority.” In the fourth century the church began to use it to mean “the accepted list of books of inspired Scripture.”
2. Carl F. H. Henry, God, Revelation and Authority, Vol. 4 (Carlisle: Paternoster, 1999), p. 407.
3. This is now widely accepted. See for example R. Beckwith, The Old Testament Canon of the New Testament Church (London: SPCK, 1985), p.276. Also A. Bentzen, Introduction to the Old Testament, Vol. 1 (Copenhagen, 1948), p. 31. Bruce Metzger, The Canon of the New Testament (Oxford: Oxford University Press, 1987), p. 110. John Wenham, Christ and the Bible (London: Tyndale Press, 1972), pp.138–139.
4. This is the general evangelical position, but John A. T. Robinson, who was not an evangelical, in his detailed scholarly book, placed the entire New Testament before AD 70. Redating the New Testament (London: S.C.M. Press, 1976).
5. Athanasius was also the first to use the word canon for the body of New Testament books.
6. Our earliest nearly complete single-volume New Testament in Greek—Codex Sinaiticus from the mid-fourth century—measures 16 by 14 inches and contains 694 pages.
7. These are called pseudepigrapha (from Greek words meaning “false writings”).

Source: https://answersingenesis.org/the-word-of-god/why-66/