What evidence is there to support Deuteronomy being written during the reign of King Josiah?

Question

I was reading an article today, and the author claimed that King Josiah sponsored the writing of Deuteronomy. Here's the exact quote:

In the seventh century BCE, when the Old Testament’s book of Deuteronomy was written under the sponsorship of King Josiah in Jerusalem, the high bar was set for veneration of the book. Josiah used the scrolls written by his ‘deuteronomists’ to solidify a covenant between the Jewish people and God – and, in an inspired act of political strategy, to legitimise his legacy and claim to the land.

Here's the original article.

What evidence is there, both biblical and extrabiblical, to support the claim that Deuteronomy, the fifth book of the Law, was written during the reign of King Josiah?

Answer 1

I am not an expert on dating of Old Testament books, but I believe I can add some insights into some of the evidence relied on by critical scholars in dating Deuteronomy to the reign of King Josiah. My approach will be to narrow the date down to a general period, then identify why it was likely written at this time.

1. The Book of Deuteronomy was written in Middle Biblical Hebrew (MBH), which places the authorship broadly in the period of the late monarchy, but perhaps a little earlier than 722 BCE or even as late as the Babylonian Exile. The Priestly Source, believed to have written during the Exile or shortly afterwards, also used MBH, but occasionally showed a tendency towards Late Biblical grammatical forms. On the other hand, Deuteronomy is not a redaction of an Early Biblical Hebrew (EBH) version, because the biblical redactors and scribes always seem to have kept EBH writing whenever it does occur.
2. The Book of Deuteronomy uses the same literary style as the books now known as the Deuteronomic History (Joshua, Judges, Samuel, Kings) which forms a coherent literary unit, with each book dovetailing nicely into the next. The Book of Kings ends after the time of Josiah, which would superficialy rule out an author in the time of Josiah, but scholars believe the final chapters were added during the Exile. This places the authorship of Deuteronomy and the History towards the end of the late monarchy and therefore around the time of King Josiah.
3. A theme of Deuteronomy is the centralisation of worship. The Jewish Virtual Library entry on Josiah says "Centralization of worship is the great innovation of the Book of Deuteronomy." We read in 2 Kings that centralisation of worship was a key reform undertaken by Josiah. So Josiah and the Book of Deuteronomy are of one mind.
4. 2 Kings 22:8 describes how the "book of the law" was supposedly found in the temple during repairs. Since this is fairly universally regarded as an early version of Deuteronomy (chapters 5-26 and 28 are often nominated), At the very least this means the book existed at the time of King Josiah.
5. The reign of a predecessor, Manasseh, has been proposed, but Manasseh was a polytheist (see 2 Kings 21:5: "And he built altars for all the host of heaven in the two courts of the house of the LORD.") and would have provided no support for a monolatrist book.
6. The circumstances suggest that the book was placed in the temple shortly before it was 'found', in order to be discovered and verified as ancient. This would not have been the only time that a book was placed in the temple in order to disguise its provenance. A very similar case is reported just a few years later, in the Book of Jeremiah, which suggests the priests knew how effective such a strategem would be. Bernard S. Jackson says, in 'Ideas of law and legal administration: a semiotic approach', published in The World of Ancient Israel: Sociological, Anthropological and Political Perspectives (edited by R. E. Clements), page 193:

   This suggests that the scroll found in the time of Josiah, just a few years earlier, had also been a ‘plant’. In fact, Jeremiah has given us a vivid description of how it might have been done.

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In conclusion, there is overwhelming evidence that the Book of Deuteronomy was written during the late monarchy, very probably during the reign of King Josiah.

Answer 2

That authorship is one of two explanations offered by scholars

Rather than refer to a Christian source, an entry-level explanation of when Deuteronomy was written can be found at the Jewish Encyclopedia online site. (The unedited full-text of the 1906 Jewish Encyclopedia). The authors for this article are: Morris Jastrow, Jr., S. R. Driver, Emil G. Hirsch, Benno Jacob.

The author of the article you read was not making a definitive claim about authorship (his article was about books and book snobs, not about Judaism nor Christianity). He chose from one of the two options (see below) that scholarship has presented. Why he chose that is his own affair, but from the style of writing in that article, I'll offer that he made the choice for an artistic reason: it fits into the pattern of behavior he was building about 19th century and modern book snobs.

Note that "JE" in the text below refers to Jewish Encyclopedia.

Age and Authorship of Deuteronomy.

(From the Jewish Encyclopedia, see link above)
It is the unanimous opinion of modern critics that Deuteronomy is not the work of Moses, but that it was, in its main parts, written in the seventh century B.C., either during the reign of Manasseh, or during that of Josiah ... {emphasis mine}

The reasons {evidence} for this conclusion, stated here in the briefest outline:

1. Even upon the assumption that JE in Exodus and Numbers is Mosaic, the historical discrepancies in Deut. i-iv. and ix.-x., and the terms in which incidents belonging to the fortieth year of the Exodus are referred to, preclude the possibility of Deuteronomy being Mosaic likewise; while the use of the expression "beyond Jordan" in i. 1, 5; iii. 8; iv. 41, 46, 47, 49, for eastern Palestine, implies that the author was a resident in western Palestine.
2. The same conclusion follows, a fortiori, for those who allow that JE is a post-Mosaic document, from the fact, noticed above, that JE itself, both in the narrative parts and in the laws, is repeatedly quoted in Deuteronomy.
3. In Deuteronomy it is strictly laid down that sacrifice is to be offered at a single central sanctuary (xii. 5, 11, 14, etc.); whereas in Joshua to I Kings vi. sacrifices are frequently described as offered in various parts of the land (in accordance with the law of Ex. xx. 24), without any indication on the part of either the actor or the narrator that a law such as that of Deuteronomy is being infringed.
4. The other differences between the legislation of Deuteronomy and that of Ex. xxi.-xxiii. point with some cogency to the conclusion that the laws of Deuteronomy originated in a later and more highly developed stage of society than the laws of Exodus.
5. The law of the kingdom (xvii. 14-20) is colored by reminiscences of the monarchy of Solomon.
6. The forms of idolatry referred to—especially the worship of the "host of heaven" (iv. 19, xvii. 7)—point to a date not earlier than the reign of Ahaz, and more probably to one in the seventh century B.C.

The USCCB's (Conference of Catholic Bishops) on line version of the New American Bible offer this in the introduction to the book of Deuteronomy:

The book was probably composed over the course of three centuries, from the eighth century to the exile and beyond. It bears some relation to “the Book of the Law” discovered in the Jerusalem Temple around 622 B.C. during the reign of King Josiah (2 Kgs 22:8–13). It gives evidence of later editing: cf. the references to exile in 4:1–40; 28:63–68; 29:21–28; 30:1–10.