There is, I think, I fairly common view among Christians (and especially conservative Christians) that the Bible with all of its various books and authors, is describing a single overall truth. Sure, the Old Testament may appear to have a different sort of theology than the New Testament, but that (in this interpretation) reflects the ongoing work of God and his plan for humanity. Sure, there may appear to be conflicts among various New Testament writers, or even among various writings by the same author. These are taken to be indications that the underlying truth may be subtle, and that further study will resolve these apparent difficulties by uncovering a unified underlying framework. This kind of outlook is particularly likely, I think, among those that believe the Bible is the inspired, infallible word of God.
I will not pretend to know exactly how common this type of view is, but I can say that it would have described my own views on the subject for most of my life. However, as I began to re-evaluate my beliefs, I encountered compelling reasons to discard this view in favor of another. In this alternative view, the various books of the Bible reflect the understandings and goals of their authors, understandings and goals which change over time and which are not mutually consistent. Under this view, it is particularly important to identify the context in which the authors wrote, and even who they were (or were not).
Perhaps the most important reasons derive from the development of the New Testament canon. (Perhaps I should say canons, since there are several, but I will focus just on those 27 books that constitute the canon with which most Catholic and Protestant Christians are familiar.) There were many other Christian writings that were not accepted into the canon. How did we get the collection that we have today? Why should we give them particular trust and exclude the others? A common answer given by Christian apologists is that three factors were critical: apostolic authority, doctrinal correctness and widespread use. Apostolic authority means that the books were written by apostles, or by close associates of the apostles in the cases of Mark and Luke. Doctrinal correctness and widespread use seem like fairly obvious criteria, but I will return to them in a minute.
We know of a great many early Christian writings that claimed to have been written by apostles. The Gospel of Judas was in the news last year about this time, having been recently found and translated, but there were many others that claimed Paul, Peter, Thomas, James, Barnabas, Mary Magdelene and even Jesus as authors. All are agreed by modern scholars to be pseudonymous; that is, they were not written by the people that they claim as authors. So, based on the criteria suggested above, they were correctly excluded from the canon, at least the canon accepted by the Catholic church and inherited by Protestants.
However, scholars in the last several hundred years have developed strong cases against apostolic authorship for many of the writings that were included in the canon, including all of the gospels and as many as seven of the epistles normally attributed to Paul, and several others. Some of the cases are stronger than others.
For example, the pastorals (I and II Timothy and Titus) are among those attributed to Paul that are almost certainly forgeries. Evidence for this derives from writing style, vocabulary, doctrine and historical context. Over one third of the words used in the pastorals are not found in any of the other Pauline epistles, but they are common among second century Christian writings. As an example of doctrinal differences, in the uncontested Pauline epistles there is nothing like the misogynistic doctrines described in I Timothy 2:9-15. (I Corinthians 14:34-35 contains similar language, but this appears to be a later scribal insertion, based both on the content of our earliest manuscripts and the lack of continuity with the surrounding verses.) Regarding historical context, the pastorals concern churches with well-developed hierarchical structures, structures which had not yet developed in Paul's time, but consistent with early second century authorship.
Without going into any more detail here, let me just list the books whose apostolic authorship is seriously questioned by modern scholarship: Matthew, Mark, Luke, John, Acts, Ephesians, Colossians, II Thessalonians, Hebrews, James, I and II Timothy, Titus, I and II Peter, Jude, and Revelation. As I said, some of these are more certain the others to be pseudo-epigraphical, but that is more than half of the New Testament books. And these conclusions were not reached by scholars out to destroy Christianity, but generally by Christian scholars seeking to understand the documents that lie at the foundation of their beliefs.
Doctrinal correctness is another interesting issue. The development of the canon was triggered by competition between various doctrinal positions. The first canons were developed in the second century by Gnostic Christians, with "orthodox" Christians responding with their own canons, eventually leading to the canon accepted today. I put the scare quotes around "orthodox" because the question of what is orthodox and what is not is an important question here. While historically one position "won" and came to be called orthodox (which means "right doctrine"), does that really mean those doctrines are correct? Or were they just more popular? How do we know what are the correct doctrines? We cannot appeal to scripture, not when the decisions about what to consider scripture were based on doctrinal correctness to begin with!
If we fall back on apostolic authority we encounter trouble quickly enough due to the questions of what was really said by the apostles. If we fall back on inspiration by God apart from apostleship, who is to say what was inspired by God and what was not? The heretics (as judged by what became the orthodoxy) also claimed inspiration. We cannot even rely on mutual consistency because, whether or not there is some as yet unknown unifying underlying theme, some apparent inconsistencies are as yet unresolved.
Are we left then simply with widespread use? From what we can tell today, the apocryphal Gospel of Peter was more widely used than the Gospel of Mark. It was discarded due to passages that could be used to support the Docetic "heresy". As another example, The Shepherd of Hermas was also widely read but eventually excluded from the canon. As far as I know, this was due to explicitly non-apostolic authorship. So, widespread use alone is insufficient.
So, while perhaps well-intentioned, the criteria of apostolic authorship fell victim to forgeries. The forgeries were probably intended to lend apparent apostolic authority to competing doctrinal positions. For doctrinal correctness to be useful as a criteria, an external mechanism for determining doctrinal correctness would be required. Since apostolic authority was already undermined by undetected forgeries (at the time), apostolic authority could not have really been that mechanism. Widespread use was clearly viewed as insufficient, since that criteria was trumped by lack of apostolic authority and by doctrinal issues. (One might make the case that widespread use could have been sufficient for inclusion in the canon, but in any case, it would be insufficient grounds for judging the texts even probably true.)
Where does this lead? I think it causes grave problems for anyone who grounds their beliefs on the authority of scripture, particularly if their view of scripture includes the entire New Testament. As I said at the beginning of this post, that describes some Christians, but not all. That is, some Christians hold different views about the role of the Bible in forming and supporting their beliefs. By itself, these considerations do not imply that now-orthodox Christian doctrines are false or that New Testament accounts of Jesus and the early church are incorrect. They do highlight some of the reasons that these doctrines invite critical examination and that the supposedly historical accounts should be viewed suspiciously.