The JEDP theory basically states that the first five books of the bible (the Pentatauch) were not written by a single person, but rather by four different people.
I'm trying to understand if this theory is even remotely valid or if it is complete nonsense.
Specifically (to avoid this being closed as argumentative), what are the most reasonable supporting arguments for this theory? Also, what are the major criticisms against this theory?
The JEPD theory was developed in three stages, which I summarized in my answer to What was the reasons for documentary hypothesis?. It isn't complete nonsense; it's the culmination of several centuries of Bible scholarship. Whether it is a giant leap forward or a brilliant mistake, I'm not certain.
On the one hand, it's rational, and it fits different types of writing into neat categories. It has a ready answer for things that might be hard to explain otherwise.
On the other hand, it's highly speculative. I've found that it is trivially easy to look at any source text and find inconsistency. I myself have tried this method and "discovered" evidence of multiple sources in Richard Dawkins' The God Delusion.
Perhaps the most serious scholarly argument against the JEPD hypothesis is that it doesn't explain why a later editor would bring all these sources together, if the traditions had kept them separate. The more likely scenario, according to this argument, is that the doublets and stylistic differences were part of a tradition that began with Moses and accumulated over the years.
The classic works on the Documentary Hypothesis (DH) are Julius Wellhausen's Die Composition des Hexateuch und der historischen Bücher des Alten Testaments and Prolegomena to the History of [Ancient] Israel (the latter of which, incidentally, is available in English for free via google books). The theory is generally attributed to Karl Graf, but it was Wellhausen who brought it to the forefront of biblical scholarship.
The basic premise of the Prolegomena is the identification of certain contradictions throughout the Pentateuch that point toward a progression toward the "Judaism" of the Exilic and second Temple period. The arguments are very nuanced, but Wellhausen's attention to detail is staggering. Central to his argument is the apprent change of opinion regarding the centrality of Jerusalem for the worship of Yahweh. The earlier strata seem unconcerned by, for instance, Solomon (or Samuel, Elisha, etc.) making sacrifices to God outside of Jerusalem, which is strictly forbidden in the later D and P sources. Moreover, none of the above mentioned people were Levites who were the sole practitioners of the temple cult.
Biblical scholarship has come a long way since Wellhausen, but one would be remiss to toss out his work as passe or otherwise "disproven." The DH is broadly accepted by nearly every serious scholar of the Hebrew Bible (with modification and individual nuance, of course) for good reason.
Don't accept (or reject) the theory just because someone on the internet said so; read the first few chapters of the Prolegomena (with a Hebrew Bible next to you if you can)--the man was truly brilliant.
Post Script: Incidentally, Wellhausen was a devout Christian--keep that in mind when you read about his theories.
The documentary hypothesis has a number of critical shortcomings that one should be aware of:
There is no archaeological, historical or extra-biblical support for the J,E,D,P texts ever actually existing. If they were separate texts that were later compiled into one, you would think that remnants of such texts would still exist or ancient writers would have referred to them but there is zero evidence that they ever actually existed.
The JEDP theory has almost as many versions as there are proponents of the theory. So there no single theory and the different versions disagree with each other. Many of the original versions been abandoned completely. This is because there is almost an infinite number of ways to separate the Torah or any similar book - by the names of God, political view points, word choice, subject matter etc.
One of the main underlying purposes of the theory is to claim that the events of Torah never happened. They were made up or constructed by a redacted who combined other texts into a single narrative. But archaeology excavations in Israel have confirmed the historicity of the bible beyond a reasonable doubt at this point.
The main reasoning for the theory is that some passages prove that Moses could not have been the sole author, like the passage that record his death in Deuteronomy and others. This is an important point. But kings and Emperors almost never actually hand wrote their own letters. They used scribes to dictate them and to write many versions that the king told them to change until a final version was approved by the king. It is almost certain that this would be the case with Moses too, who was the leader of 200,000 - 2 million people. Scribes almost certainly did the actual writing for other books like Joshua, Kings, Samuel etc. The Old Testament books are named for the main protagonist, who could have had lots of control or little control over the exact words written by the scribes. Finally, there is evidence in the biblical text itself that King Hezekiah had all the separate scrolls or books put together into one unified work which is now the Old Testament. It would be totally natural for the final compiler to add a few transitory comments or verses to give future readers the full picture, like recording the death of Moses or saying "and we still can't locate his burial place to this day" or whatever. All of that is totally normal and expected. God did not write the Bible himself and drop it in someone's lap. Nor did he dictate it word for word as if Moses was channeling a spirit or something. With the exception of the 10 commandments which God may have dictated to Moses in almost their full form and then he etched them into the tablets.
The documentary hypothesis is not scientific because it cannot be falsified, nor can it be proven true (unless someone digs up the actual J, E, D, P texts that they allege existed at some time). It's all just literary criticism, categorization, 100% speculation and ultimately an attempt to re-write history that has no historical support until the theory was invented in the 1800's.
One has to mention the motivations and beliefs of the academics pushing this theory. Given that there is no historical or archaeological evidence for the theory and that there is no consensus even among those scholars who support the theory, and by the very nature of the theory it is impossible to falsify or verify, I would take it with a grain of salt.
Strictly speaking, this is not a theory, but a hypothesis. In scientific terminology, a theory is a well-substantiated, unifying explanation for a set of verified, proven hypotheses. A hypothesis is a suggested explanation for an observable phenomenon, and this is what JEDP is. The Documentary Hypothesis, as originally proposed by Wellhausen, is no longer accepted by the majority of biblical scholars as definitive but, with various proposed modifications under consideration, it remains the best explanation we have for the development of the Pentateuch.
The only serious alternative to the JEDP hypothesis is the tradition that Moses was the author, or at least the main author, of the Pentateuch. For this to be a viable explanation, there ought to be evidence that Moses was a real, historical person and that he was writing from his own experience. This evidence that must be found outside the Pentateuch itself, and could be found by comparing the extensive Egyptian records that we have, against the Book of Exodus. However, Lester L. Grabbe says in Ancient Israel, page 85, there is nothing in Egyptian texts that could be related to the story in the Book of Exodus. Carol A. Redmount says, in 'Bitter lives', published in The Oxford History of the Biblical World, page 63, the biblical Exodus account was never intended to function or to be understood as history in the present-day sense of the word. Perhaps there is an underlying historical core in Exodus but we should expect that a participant in the Exodus would write an account that more closely reflects his experience.
Mark S. Smith says, in his preface to The Early History of God (page xxiii), the older source theory of the Pentateuch (the Documentary Hypothesis) had already come under serious fire when The Early History of God first appeared (First edition: 1990). He says the newer redactional model developed by E. Blume and extended by D. M. Carr on the biblical side, and studies of redaction in Gilgamesh by J. H. Tigay on the ancient Near Eastern side, have complicated source theory without abolishing it. While the death knell for source theory was sounded often over the course of the 1980s and 1990s, it has not been supplanted by a more persuasive model.
Joel S. Baden says, in 'The Re-Emergence of Source Criticism: The Neo-Documentary Hypothesis' that European scholarship abandoned the JEDP hypothesis as American scholars continued to support it. He says this situation is changing, as the Documentary Hypothesis is regaining its place as a viable, productive, and current approach to the Pentateuch. One of the main contributions of more recent source-critical work has been the identification and correction of the methodological problems that plagued earlier scholarship, contributing significantly if not primarily to the move away from the Documentary Hypothesis in Europe in recent generations.
Here is, according to the traditional view, the most likely explanation behind how the Torah was compiled.
Traditionally, the Torah is held to be a divinely authored and prophetic work by Moses (John 5:46–47). However, some parts of the Torah are impossible to have been written in one setting, because they describe events that occur during the 40 year sojourn in Sinai.
What most likely happened was that when Moses went to Mt. Sinai for 40 days and nights (Exodus 24:18) he received three sets of information: the history of the old world (the Book of Genesis, which is like a preamble to the Torah), the history of the Exodus and the time spent in Egypt (the first half of Exodus) and the moral and religious ordinances (the 10 commandments, but also most of the laws of the Book of Leviticus, in addition to details on how to build the tabernacle (Exodus 31:11)).
These sets were handed over to Aaron, who was responsible for compiling the religious parts into the Book of Leviticus (which is why the only history included in Leviticus is the family history of Aaron's sons Nadab and Abihu) and Joshua, who compiled the rest of Exodus and Numbers, recording in real time events like the Rebellion of Korah (Numbers 16:1–40) and the cowardly spies (Numbers 13:1-33).
Lastly, Moses, before he died, offered a final speech which became the Book of Deuteronomy, which is why it is the most song-like of the five books and the most unified in structure. Lastly, the Book of Deuteronomy is signed off with Joshua's description of Moses's death (Deuteronomy 34).
This concludes the most likely explanation for how the Torah was compiled.
