What is the most important book in the Old Testament? Those with a high view of Scripture will argue that there is no “most important” book. Indeed, “all Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness” (2 Timothy 3:16). In this sense, no single book of the Old Testament—or, indeed, of the Bible—is more important than another.

At the same time, it is fair to say that some of the books of the Bible, while important in their own right “for teaching, for reproof, for correction, and for training in righteousness,” do not, in and of themselves, further the storyline of the Bible.  So, for example, it is possible to understand the overall theme of the Old Testament without an intimate knowledge of Obadiah or Zephaniah. While one certainly does want to understand the overall purpose of the Minor Prophets, it is possible to overlook some of the individual books in seeking to gain a good overview of the Old Testament.

There are certain books, however, for which this cannot be done. One cannot, for example, gain a healthy understanding of the basic message of the Old Testament without Genesis. In seeking to understand the basic message of the Old Testament, Genesis, it can be argued, is more important than Zephaniah.

But which of the books is the most important? This question doesn’t have one correct answer, but if you consider the Old Testament as a unit, the book of Deuteronomy must surely figure heavily in your priorities.

What is Deuteronomy?

The word “Deuteronomy” literally means “second law.” It is derived from the Greek words deutero (second) and nomos (law). It is the “second” law because the exodus generation had all died in the wilderness, and Deuteronomy was given to a new (second) generation as they prepared to enter the Promised Land.

First unilateral, now bilateral

Deuteronomy emphasises the bilateral nature of the covenant. God’s original promise to Abraham in Genesis 12:1–3 did not contain the word “if.” God promised unilaterally to do some things for Abraham and his descendants. Until this point in the Pentateuch, the covenant has remained largely unilateral, but now things change.

Deuteronomy emphasises the “if/then” nature of the covenant (see, for example, Deuteronomy 30:11–20). If the people obeyed, they would be blessed; if they disobeyed, they would be cursed. And it is this bilateral nature of the covenant that is so crucial to a healthy understanding of the remainder of the Old Testament.

The bilateral nature of the covenant explains why the faithful generation conquered Canaan under Joshua and why their children failed to complete the conquest in Judges. The first generation was faithful to God’s covenant (Joshua 24:31), and so received God’s blessing. Their children, however, were not faithful to complete what they had started (Judges 1:27–36). Instead, they gave themselves to idolatry and thus inherited God’s curse (Judges 2:11–15).

Similarly, Deuteronomy sheds light on David as the ideal king. David is known as a man after God’s own heart (1 Samuel 13:13–14; Acts 13:22), but this was not an assessment of his personal purity. Indeed, he was an adulterer, a murderer, and largely a failure as a father. If he lived in the new covenant, he would not have fulfilled the qualifications for eldership in the local church! But he was a man after God’s own heart in that he led the people in covenant faithfulness. This assessment makes sense in the light of Deuteronomy.

Similarly, Solomon’s failure must be understood in the light of Deuteronomy. It wasn’t his polygamy per se that resulted in the division of the kingdom. Instead, those foreign wives turned his heart from God (1 Kings 11:1–3ff) so that he abandoned covenant obligations that God’s curse fell upon the nation. The division of the kingdom only makes sense in the light of Deuteronomy.

Deuteronomy explains the constant friction between prophets and kings in the kingdom era. It explains why things went well with Israel during the reign of covenantally faithful kings, and poorly during the reign of covenantally unfaithful kings. Ultimately, it explains the exile itself. Simply put, everything from conquest to exile only makes sense in the light of Deuteronomy.

Tension arises

And yet this creates something of a problem. It appears that unilateral and bilateral covenants cannot exist in harmony. A covenant, so we think, must be either unconditional or conditional; it cannot be both.

As you read the Deuteronomy record, you come away with a distinct sense of hopelessness. There is just no way that Israel can keep this covenant. It seems that the “second law” condemns people without offering them hope. In fact, this appears to have been the opinion of God himself (see Deuteronomy 31:16–21).

And yet there is a wonderful promise: “Rejoice with him, O heavens; bow down to him, all gods, for he avenges the blood of his children and takes vengeance on his adversaries. He repays those who hate him and cleanses his people’s land” (Deuteronomy 32:43).

When God restated his covenant with Abraham in Genesis 15, he very clearly took upon himself the responsibility for both sides of the covenant. In ancient times, when a covenant was made, an animal was slaughtered and divided in two. The parties to the covenant placed the halves of the animal opposite each other, and together walked between the pieces, thereby signifying that either party breaking his part of the covenant was worthy of the same fate as the animal. But when God reiterated his covenant with Abraham, he put Abraham to sleep and passed between the carcases alone. Though there would be obligations, God would meet those himself.

In Deuteronomy, we learn that failure to adhere to the covenant obligations would result in a curse for God’s people. And neither God (Deuteronomy 31:16–18), Moses (Deuteronomy 31:24–29), nor Joshua (Joshua 24:14–21) were confident of the people’s ability to obey. It was certain that they would fail.

Even Moses, greatest of Israelite prophets (Deuteronomy 34:10–12) and mediator of the old covenant, failed and died for his unbelief (Numbers 20:12–13). But not all was hopeless. Indeed, God would raise up a better prophet than Moses (Deuteronomy 18:15–19) who, though innocent, would die the death of a worthless son, being hanged on a tree and cursed by God, so that those who deserve God’s Deuteronomy curses mighty instead experience his Genesis blessings.