I have heard this statement many times. It can come from Christians or non-Christians, but mainly I hear it from unbelievers this idea that the Bible is inadmissible as evidence for itself. If I were trying to use the Bible to prove the validity of the Bible (from the perspective of many outsiders), this is circular reasoning. This statement is not only wrong, but completely misunderstands its own argument; ironically, it makes the exact circular assumptions that it accuses believers of.
1. The “Bible” is not one book
When we are talking about “proving” or evidencing the truths of the Gospel message, we have to put our historian hats on (not our religious hats). The argument is meant to place Christians in this rather odd situation where they sound like they are saying the Bible is true because it says it is true. But the Bible is not one book. In fact, the term “Bible” is not in the Bible. The Bible is a collection of works that spans over a thousand years, written by dozens of authors, some who are connected, some who are not. All together there are sixty-six books in the Protestant Bible.
When we are talking about the claims of the “New Testament,” we are talking about the story of Christianity, the very foundation and apex of Christianity as it deals with the incarnation of Christ, who he was, and what he did. But even then, to say one can’t prove the New Testament with the New Testament is quite ill-informed and unreflective. The designation “New Testament” (along with its list of books) is not even in the New Testament. Like with the whole Bible, it is just a name given to a certain related corpus of writings that speaks about the story and implications of the advent of Jesus Christ. There are twenty-seven books in the New Testament.
If one were to look at this with a historian’s eye, to say we cannot use the Bible to prove or evidence the Bible is about the most misguided thing one could possibly say. What does that mean? Are you saying that we cannot use the testimony that the book of Matthew gives to evidence Mark? Or that one cannot attempt to piece together Galatians with the Book of Acts? Of course you can. In fact, you must. These twenty-seven documents, all written around the same time, all telling similar stories, must be used to prove or evidence each other. If not, the historian is not being a historian, but something entirely different.
2. One must assume the inspiration of the Bible to say the Bible can’t prove the Bible
You see, if a person says, “You can’t use the Bible to prove the Bible,” he probably doesn’t realize he is borrowing a bit from the Christian worldview in order to even make such an assertion. What is being borrowed? The idea of the basic unity of Scripture or the single-authorship of the Bible. The only way to say the Bible can’t prove the Bible is to presume the inspiration of Scripture. Otherwise, there is no reason to link the canon of Scripture together in such a way. For the non-Christian especially, the Bible should be seen as sixty-six ancient documents, all of which stand or fall on their own. In order to make them stand or fall together, one must assume a single authorship of some sort. At that point, the argument becomes self-defeating, as the very statement (“You can’t use the Bible to prove the Bible”) proves the Bible!
3. Most events of ancient history have no more than one contemporary witness (if that many)
The twenty-seven ancient documents called the New Testament are unparalleled in ancient history as far as their testimony. The contemporary multiple attestations for the story of Jesus (eyewitness or not) are without equal. Look to the sources we have for other ancient historical events and people, and you will find that they have nowhere near the amount of documented writings discussing the central claims.
Yet when it comes to the claims about Christ, we are talking about twenty-seven documents in the New Testament alone! And all of these come within sixty to seventy years after the events. And if you expand the data beyond just Scripture and allow extrabiblical sources to be considered, then we are talking about dozens and dozens more from early church fathers (whose testimonies cannot be ignored simply because they believed; what if we did that with the seemingly miraculous landing on the moon? “All those who do not believe it happened, step up to the evidence table!” Uh, no.) and from ancient historians such as Tacitus and Josephus.
In the end, the story of Christ has plenty of independent documentation, all of must prove or evidence the rest. So in this sense, we must use the Bible to prove the Bible or else we are not being historians, but religious zealots, fighting to keep hold of our unbelief through stupid statements.