How can I be sure that the right books were included in the bible?

Your Bible contains sixty-six books. Someone had to know which writings belonged in it and which did not, and if that judgement went wrong, everything resting on it is unsafe.

The early churches did not select those books. They recognised them. The word canon comes from a Greek word meaning a rule or a measuring rod, and applied to the Bible it means the list of books recognised as the word of God — recognised, not chosen. Bruce Metzger, who spent his career at Princeton Theological Seminary studying how the New Testament came together, drew the distinction as sharply as it can be drawn: the canon is a collection of authoritative books, not an authoritative collection of books. Think of a jeweller examining a diamond. His certificate does not make the stone valuable. The stone was already what it is, and his work is to identify it correctly and say so. No book became God's word by being listed. It was listed because it already was.

That raises the obvious next question. If they were identifying rather than deciding, how do we know they identified correctly?

For the Old Testament, the answer is that the early church never had to. Jesus and His apostles received the Hebrew Scriptures as a body already given and already settled among the Jewish people. Christ did not argue for its contents or trim its edges. He spoke of it as a single collection with three familiar parts.

Luke 24:44 (NKJV)

Then He said to them, “These are the words which I spoke to you while I was still with you, that all things must be fulfilled which were written in the Law of Moses and the Prophets and the Psalms concerning Me.”

The Law, the Prophets and the Psalms was the standard Jewish way of naming the whole Hebrew Bible, and those are the same thirty-nine books your Old Testament contains today. Jesus treated that collection as the words of His Father and expounded it as such. Whatever else is disputed about the canon, the Old Testament rests on His verdict, not on the church's. The additional books found in Roman Catholic Bibles raise a separate question, taken up in the next chapter.

The New Testament is where the work of recognition happened, and it happened by tests the churches applied consistently rather than by anyone's decree. The first was apostolic origin. Christ appointed apostles and gave them His own authority to teach, so a writing had to come from an apostle or from a close companion writing with apostolic backing — Mark, who travelled with Peter, and Luke, who travelled with Paul.

The second test followed from the first. The Christian message is not a philosophy that could be worked out in a study. It is a report of things that happened — a man born, crucified, buried, and raised again, in places you can still visit and under a governor whose name survives in the records. A claim of that kind can only be carried by people who were there. So the churches asked whether a writing came from a man who had seen these events himself, or from one who had gone to those who had. A story that reached the page through several hands, decades removed from anyone present, was not received, however edifying it sounded.

The third test was the agreement of the churches. By the second century Christian congregations were scattered from Gaul and North Africa to Egypt, Syria and beyond, separated by distance and by persecution, speaking different languages, with letters carried on foot or by ship over weeks and months and no bishop or council holding authority over the rest. Those separated churches read the same four Gospels and the same letters of Paul. Communities that far apart, with that little contact, do not arrive at the same list by coincidence; the agreement had a cause outside of them, and that cause was the character of the books themselves. So a writing received in one place and nowhere else was held under suspicion, while a writing the churches had received everywhere had already shown what it was long before any council met to say so.

The fourth was public use. A book read aloud to a whole congregation, week after week, was a book exposed to challenge. In cities where men and women who had known the apostles were still alive, a forgery read from the front could be contradicted on the spot by someone who knew better. That is why the writings that failed tended to circulate privately instead, among small groups who claimed a knowledge ordinary Christians did not have. They avoided the one setting in which their claims could have been tested.

The fifth test was doctrinal consistency. God does not contradict Himself, so a writing that cut against what the apostles preached was ruled out no matter whose name was attached to it.

This recognition was already happening while the apostles were still alive. Paul thanked the Thessalonians not for admiring his preaching but for receiving it as what it actually was.

1 Thessalonians 2:13 (NKJV)

For this reason we also thank God without ceasing, because when you received the word of God which you heard from us, you welcomed it not as the word of men, but as it is in truth, the word of God, which also effectively works in you who believe.

Peter did the same with Paul's letters, setting them alongside the rest of Scripture without argument, apology, or any sense that he was granting them a status they lacked.

2 Peter 3:15–16 (NKJV)

and consider that the longsuffering of our Lord is salvation—as also our beloved brother Paul, according to the wisdom given to him, has written to you, as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures.

The process was not instantaneous. A handful of books — Hebrews, James, 2 Peter, 2 and 3 John, Jude and Revelation — were discussed longer than the rest, usually because their authorship was less obvious or their early circulation narrower. That delay is a mark of care rather than confusion. Nobody was waving books through. They were being weighed, and the weighing took time wherever the evidence took time to gather. Nor does anything in the Christian faith hang on those seven alone; every doctrine they teach is taught again in books that were never questioned by anyone.

The writings that failed the tests failed them badly. The Gospel of Thomas, recovered in Egypt in 1945, borrows the name of an apostle its author had no right to use. The Gospel of Judas, published in 2006, does something stranger: it takes the name of the man who sold Christ for silver, and recasts the betrayal as obedience to a secret instruction the other disciples were too dull to receive. Scholars date both works to the second century, well after the men named in their titles were dead, and Irenaeus was already writing against a Gospel of Judas around AD 180. Their contents settle the matter faster than their dates do. Both teach salvation through hidden knowledge reserved for an inner circle, which is the exact opposite of a gospel preached openly to the whole world by men who died rather than retract it. These were not suppressed books. They were late books, examined and rejected on their merits.

None of this leaves your confidence resting on the competence of ancient churchmen. If God took the trouble to speak, He is not the kind of God who then mislays what He said. The same providence that moved men to write moved His people to recognise what had been written.