This is an incredibly oversimplified explanation, which leaves out most of the scholarly debate and hard work done by generations of great minds. I stand behind everything you’re about to read, though it is not an academic treatise and truly leaves out many of the debates and research that got us here. It is a photo of the finish line, not the entire race.
Before I get into any details about how early church leaders gave us a complete Bible, I want to establish that the Bible is from God. God is the author, and 40 people were writers. The criteria and efforts employed by church leaders and scholars in antiquity did not “decide” what was in the Bible; God did that. What these leaders did was come up with a process that recognized what was from God, and clearly identified what was not. God authored the Bible; all the words in the Bible are true.
How did we come to accept the specific 39 books of the Hebrew Scriptures (Old Testament) and 27 books of the (New Testament) to be the Bible? And who had the authority to make these decisions? Each of the 66 books in our Bible was approved by leaders in the ancient world, with many criteria ultimately determining whether each writing could be traced back to trusted sources and authorities, such as the Apostles and their converts.
First off, we call this Canonicity. The “canon” derives from the Greek word κανών which is essentially a word that means “measuring stick” like a ruler or yardstick. The word is used to describe the books that have been properly measured and tested as authentic and are considered Scripture. In the ancient world, there initially was no need for a correct “list” of Scripture, but the arrival of false writings and heretical materials caused leaders to finally give an endorsement in writing to which books were already considered authentic to defend against cults as well as confusion.
Many early writings exist that speak matter of fact about particular books being considered Scripture. There were other materials that early Christians considered valuable and important in existence, but they stopped short of considering them Scripture. Examples of this would be a group of 15 books used by The Roman Catholics, Eastern Orthodox, and a few other traditions called the “Apocrypha” as well as some other early writings. These writings are quite interesting but they are not Scripture. The criteria used by the ancient church to determine which books were considered Scripture consisted of these 5 tests: Is it authoritative? Is it prophetic? Is it authentic? Is it dynamic? Is it received (consensus)?
For a greater amount of detail on how the early church used both external and internal evidence to determine which books were Scripture and which books were not, there is a brief bullet list or criteria listed here on the Bible.org website that I highly recommend.
Here is a summary of who/how the Old Testament was established as authentic, as well as how most of the New Testament can be validated by simply using the words of the New Testament writers, aside from the criteria mentioned above. Fascinating stuff.
The Ancient Hebrews. The Hebrew Bible (Biblica Hebraica) contains the 39 books of the old Testament. Protestants have accepted this canon as the entire Old Testament. Some organizations including the Roman Catholic Church and Eastern Orthodox have used other criteria to admit additional writings such as the Apocrypha into their Bible, but we have seen fit to allow the Hebrews to be the authority on their own Scriptures. Again, the Apocrypha are very important and quite interesting writings, but they simply are not Scripture. They are not “bad” books and I am certainly not against anyone reading them at all. I’m just not putting “Bel and the Dragon” on the same pedestal as the book of “Daniel.”
Peter, Paul, Matthew, James, and John. There are 27 books in the New Testament. Let’s see what “internal evidence” within the Bible says about each of these books. Jesus Himself said of the Apostle Peter “…on this rock I will build my church…” Peter took this endorsement and wrote the books of 1 & 2 Peter. That brings us down to 25 books. Peter himself said Paul’s writings were a part of the Scriptures (2 Peter 3:15-16). Paul wrote 13 books so that cuts our task down considerably but still leaves us with 12 books. Well, Matthew and John were Apostles, and they account for 6 books together. That means 6 to go. James was the brother of Jesus (well, technically his half brother) which gives us 5 books. Paul Himself claimed Luke’s writings were also Scripture, which leaves us with our last 3 books: The Gospel of Mark, The Book of Hebrews, and the book of Jude.
The rest may bore you, but here are just a few more minor details about how we came to accept these final three books. If you think this is overkill, the ancient church itself conducted an incredible amount of research and examination to settle the issue of “canonicity” forever, and as a result we have what we have today.
The Gospel of Mark: Among other things, two early sources (Papias of Hierapolis and Irenaeus) claim that Mark is the author of the book attributed to his name and included in the Bible. The first guy named Papias was a disciple of John the Apostle who later became a bishop, and considered it a matter of fact that Mark interviewed Peter for the content of his Gospel.
Irenaeus in a similar way was a disciple of a man named Polycarp, who is known to have been a disciple of the Apostle John just as Papias was. Irenaeus in his writings quotes most of the New Testament books, including Mark as well as the other Gospels, the book of Hebrews, and just about everything Paul wrote. While other ancient figures are vital to the painstaking process of determining which books are considered Scripture, Irenaeus (and Papias) is considered a very credible source do to his writings and close proximity in relationship to the Apostle John.
The Book of Hebrews: The identity of this book’s writer is unknown, though some have suggested it was Paul, Luke, and others, including a New Testament woman named Priscilla who may have remained anonymous due to the fact that women didn’t exist in the same station as men in ancient Greco-Roman society. Certainty of authorship for this book is fun for some to debate yet completely impossible to establish. This question regarding authorship made it one of the most scrutinized books in the Bible. It is written in a much more eloquent style than any other New Testament book. While Irenaeus referred to the book of Hebrews in one of his writings, there’s much more to it being in the Bible than some name dropping. If name dropping was enough, we’d just start with Irenaeus and add in the fact that Clement, Augustine, Origen, and Jerome all considered the book to be Scripture. Plus there is an ancient papyrus from 200 AD we call p46 that contains the book of Hebrews. All this to say that in the ancient world, this was a relatively well circulated writing.
Even more than just these historical “hall of fame” name dropping endorsements is the fact that the book of Hebrews articulates so well the Hebrew Scriptures as foretelling the person and work of Jesus Christ. Hebrews so well related the Old Testament writings to the person of Jesus Christ that in the end it did not matter who wrote the book, because it simply related what was already known about the Old Testament to the New Testament. It is a fascinating book to study.
The Book of Jude: Jude is probably another brother of Jesus, but definitely not the same Judas who betrayed Jesus into the hands of an angry mob for 30 pieces of silver. The book is short: only one chapter of 25 verses. It is similar the the second chapter of 2 Peter, which indicates that both 2 Peter and Jude may have consulted a similar source. Either way, this book was considered to be a part of the Scriptures by many of the usual suspects in the ancient world, such as Clement, Tertullian, and later a guy by the name of Athanasius. Even an early writing called “The Muratorian Canon” that served as a list of accepted books in Scripture listed Jude as one of them.
SO that’s it. That’s all 66 books accounted for, though again this is not a scholarly document. It is just a photo of the finish line, and not the entire race.