Yet, many people ask an even more fundamental question: How do we know that the 66 books included in most Bibles are truly authorized as part of the canon, the authoritative collection of inspired Scripture? How can we be sure that we have the complete Word of God?
It is a good question. Most people believe that the early Catholic Church decided which books were authentic, and we have just received the results of its decision. This, however, is not true. The Catholic Church did not authorize the biblical canon—it only accepted it. The Bible has its own internal authorization protocols that the Catholic Church merely followed, and subsequently, most others also accepted. It is evident from the agreement of the 5,000 extant, ancient copies of the New Testament that the canon was already set before the Catholic Church put its stamp of approval on it.
Actually, only a few books now regarded as Scripture ever raised any questions regarding canonicity: James, Jude, II Peter, and II and III John (all disputed due to questions of authorship). All of them are attested in early writings as authoritative. In fact, it has been shown that the “early church fathers” quoted from the canonical books so much that, if the Bible somehow ceased to exist, it could be reassembled in full, minus just a handful of verses, using only their writings. Officially, by ad 140, the visible church (we could call it the proto-Catholic Church) recognized all fourteen of Paul's letters plus all four gospels. The first historical list of all 27 New Testament books dates to ad 367. The Catholic Church did not officially ratify them (by papal decree) until ad 405.
As mentioned above, the Bible contains internal authorization protocols. The most esoteric may be the prophecy in Isaiah 8:16: "Bind up the testimony, seal the law among my disciples.” The early church understood this to mean that the canon would be "bound," that is, finished and authorized, by the time the original twelve apostles had died. The apostle John was the last of the original Twelve to die (around ad 100), and it is supposed that he gathered the present 27 New Testament books together and authorized their use in the churches.
The Bible itself provides a clue that Peter had already begun some of this canonization many years earlier (as early as the mid-ad 60s). II Peter 3:15-16 suggests that Paul's epistles had already attained the status of Scripture by that time (see another hint of a collecting of Paul’s epistles by Paul himself in II Timothy 4:13). It is easy to assume that this may also embrace Luke's Gospel and Acts (Luke was Paul’s longtime assistant). If Peter had indeed begun the canonization process, both of his epistles and the gospel of Mark (understood to be written under Peter's direction) can also be included. This now makes nineteen authorized books. Later, John would include his Gospel, Revelation, and three epistles, making a total of 24 books.
The only questionable books, then, would be Matthew's Gospel, James, and Jude—and there are no legitimate, canonical problems with them, as all three were written by apostles (two of them Jesus’ half-brothers!). This brings up another of the protocols for canonicity: The authorship of a book must be apostolic (exceptions are made for the writings of Luke and Mark, as they were considered to be written under Paul’s and Peter’s direct supervision).
Perhaps the most important protocol for canonicity, though, is what is termed “internal unity,” sometimes called conformity to the "rule of faith." It is evident that the New Testament books agree on doctrine, Christian living, history, and prophecy. They contain internal unity; they are a whole in 27 parts. Other books or epistles—for instance, the Gospel of Thomas or the Epistles of Clement, which have been suggested as canonical—disturb this unity. Many books have been written to show that the canonical Bible does not contradict itself, particularly in areas of doctrine.
A final rule of canonicity is general acceptance by the church. While there were differences among the congregations about which books were to be read in the churches, they all agreed on these 27 books. Eventually, the others were found wanting, and the present 27 were authorized. Again, we should note that all this took place before the rise of the organized Catholic Church in the second century.
The 39 books of the Old Testament have undergone similar tests of canonicity. A few books, such as Esther and parts of Daniel, have been questioned, but in the end, their reliability has been universally recognized. Though some churches accept the Apocrypha—the books of Maccabees, Esdras, the Wisdom of Solomon, Tobit, Bel and the Dragon, etc.—even a quick perusal of their texts proves them to be of lesser quality and worth than the accepted books. In addition, the biblical books found among the Dead Sea Scrolls attest to the precision of their transmission through the ages. Thus, scholarly debate on Old Testament canonicity has largely subsided to minor disputes on peripheral matters.
There is no valid reason to doubt the authoritative nature of the 66 books of the Bible. What has come down to us is God’s “prophetic word made more sure” (II Peter 1:19), “given by inspiration of God” (II Timothy 3:16). We can absolutely trust what is written in it to guide us along the narrow way to the Kingdom of God.