Another methodological problem with the Documentary Hypothesis, which many scholars have noted, is that it is a massive exercise in circular reasoning. How do we know if a passage belongs to J, E, D, P, or other proposed authors? The answer is: it’s complicated; very complicated. And, unfortunately, different scholars see different characteristics for the various purported authors. The more you dig into the details of the various claimed characteristics, the more complex and often seemingly haphazard the system becomes.
For example, take P, the “Priestly” author. Certain portions of the Pentateuch contain detailed description of priestly regulations and purity laws which are of interest largely to priests. The Documentarians propose that these passages were therefore written by a priest for priests. Fair enough. But why should we assume that priests don’t have interest in non-priestly matters? Or why should we assume that the Yahwist (J) author wasn’t a priest himself? Can’t a priest be interested in the history of Abraham? Can’t a priest call God by the name Yahweh? Indeed, given the sociology of scribal culture in ancient Israel, it’s quite possible that all the proposed authors of the Pentateuch were actually priests.
If the Yahwist was a priest, why couldn’t the Yahwist also be the author the P/priestly material in the Pentateuch? If the Yahwist was a priest, what criteria do we have left for distinguishing J/Yahwist from P/Priestly texts in the Pentateuch? Indeed, some Documentarians now claim that the Pentateuch should be divided into only two categories P and non-P. Others, quite the opposite, think P is merely a redactional layer, with no independent text at its foundation. Furthermore, if priests were interested in non-priestly matters—which they most certainly were—then non-P could have actually been written by a priest as well. The grounds for this division become increasingly problematic, and increasingly circular.
The argument basically devolves into this: P was a priest, and was therefore interested in genealogy, law, ritual, and tabernacle, all legitimate priestly concerns. Therefore, any passage in the Pentateuch that deals with genealogy, law, ritual or tabernacle must have been written by P.
The claim that only a priest would write about priesthood, tabernacle and ritual also ignores the possibility that non-priests might be interested in these matters. This is certainly the case texts written in later Jewish history, which often show intense interest in such matters. Jesus himself, from the tribe of Judah, was quite interested in the temple and its priests. Should we take all the portions of the New Testament that discuss priesthood and temple and assume they must have been written by a priest?