Chris M. from Houston, Texas, writes,
The late Christopher Hitchens would mockingly cite what appears to be the "second" ten commandments in the Bible in Exodus 34, that on the surface appear to be radically different than the original ten in Exodus 20. This has bothered me from a lay perspective as I haven't heard many sermons or found many commentaries explaining what this difference is all about. Can you help me reconcile these two passages, and understand which one is which and what the difference is?
We posed this question to Jason S. DeRouchie, associate professor of Old Testament and Bethlehem College and Seminary.
Through the history of Christianity, few portions of the Old Testament have influenced the church more than the Ten Words revealed by God to Israel at Mount Sinai. While often termed the "Ten Commandments," the Hebrew label preserved in Exod 34:28, Deut 4:13, and 10:4 is "Ten Words," which is also the etymology of the term Decalogue (from the Greek deka 'ten' + logoi 'words'). Nevertheless, Moses declares that the Ten Words were "commanded" (Deut 4:13), and Jesus explicitly calls them "commandments" (Matt 19:17-19), so the traditional title is not misdirected.
The importance of the Ten Words is seen in the facts that they are the first written material in Scripture specified as authoritatively binding (but note Gen 5:1 and Exod 17:14) and that they are the only portion of the Bible that we are told was "written with the finger of God" (Exod 31:18, Deut 9:10; cf. Exod 24:12, 32:15-16; Deut 5:22). They are classified as "the words of the covenant" (Exod 34:27-28), which highlights how they and all the rest of Scripture that develops from them were not the decrees of a distant Dictator but the loving instructions of a covenant Father to his vassal children, all designed to sustain relationship in the context of freedom. The Ten Words are the only part of the Bible that was placed in the ark of the covenant (Exod 40:20-21, Deut 10:1-5), and they stand in a foundational position at the head of all other instructions in Exodus and Deuteronomy. They are also echoed throughout Scripture as a summary of what it means to love God and neighbor (see Hos 4:2; Jer 7:9; Pss 50:16-23, 81:9; Matt 5:21, 19:18; Mark 10:19; Luke 18:20; Rom 13:9). All these elements display the unique role the Ten Words played among those faithful to Yahweh.
Overview of the Issue
One challenge faced by those approaching the Decalogue is the question, "Which Ten Words?" While most interpreters naturally see the title referring to the list of ten principles found in Exod 20:1-17 and repeated (with some changes) in Deut 5:6-21, critical scholars have often viewed these "ethical Decalogues" as secondary to what they believe to be the more original "ritual or cultic Decalogue" of Exod 34:11-26. In a comparable, though less developed line of thought, the late Christopher Hitchens attempted to discredit the Bible by proposing tensions between the original Decalogue of Exodus 20 and the new "re-write" in Exodus 34; he then saw the variations of the Decalogue in Deuteronomy 5 as only adding to the biblical mess. These and other approaches are highly influenced by views of biblical authority and canonical formation far different from the present author. Furthermore, a close reading of the text as it stands removes the proposed tensions and clearly designates which lists are to be regarded as the covenantal "Ten Words."
The Real Decalogue: Exodus 20, not 34
It is true that Exod 34:11-26 includes a series of apodictic principles (i.e., basic instructions without any specified contexts) and that directly after them in v. 28 the phrase "Ten Words" shows up for the first time in Scripture. However, only if one begins with v. 17 are ten directives evident, and as will be shown, Exodus 34 itself calls the reader to look elsewhere for the actual Ten Words of the covenant (34:1, 28). The prescriptions in Exodus 34 are best seen as sample laws from the Covenant Code of Exodus 20-23 (esp. ch. 23), perhaps even a festival calendar, and should not be confused with the actual Decalogue.
With respect to Exod 34:11-26, the misunderstanding has arisen because the prescriptions themselves are directly followed first by Yahweh's charge to Moses to write down "these words" in accordance with which God made a covenant with his people (a clear reference to 34:11-26) and then by the narrator's record that the "the words of the covenant, the Ten Words," were written on the tablets (34:27-28). Do we not have here a command-fulfillment sequence, wherein Moses obeys by writing the ten covenantal words on the tablets?
The answer is no, for with a back-reference to the divine activity promised and fulfilled in Exod 24:12, 31:18, and 32:15-16, Yahweh announced in 34:1 that he, not Moses, would write the same Words on the new tablets that he had written before with his own finger: "Yahweh said to Moses, 'Cut for yourself two tablets of stone like the first, and I will write on the tablets the words that were on the first tablets, which you broke.'" Yahweh, not Moses, is the antecedent to the third masculine singular verb phrase "and he wrote" in v. 28, which means that "these words" that Moses is charged to write in v. 27 (i.e., 34:11-26) are not the actual Ten Words of the covenant.
Later in Deuteronomy, Moses highlights that the Words of Exod 20:1-17 were indeed the very same Decalogue of 34:28. First, in Deut 4:12-13, the prophet states specifically that Yahweh declared his covenant, the Ten Words, out of the fire and wrote them on two tablets of stone (cf. Exod. 31:18). In echo of Exod 34:1 and 28, he then stresses in Deut 10:4 that Yawheh "wrote on the tablets, in the same writing as before, the Ten Words that Yahweh had spoken to you on the mountain out of the midst of the first on the day of the assembly." These passages make clear that the biblical author connected the phrase "Ten Words" only to the lists in Exod 20:1-17 and Deut 5:6-21.
Dealing with Differences in Deuteronomy 5
As for the relationship between Exodus 20 and Deuteronomy 5, there are clear differences in Deuteronomy's version of the Decalogue. Most obvious are (1) the change of focus from God's work in creating the world to his work in creating a people (i.e., the Exodus) as the basis for Sabbath keeping (Deut 5:12-15), (2) the shaping of the final six prohibitions into a single unit by a fronted "and" (5:17-21), and (3) the transformation of the prohibitions against coveting (5:21a-b) by using two different verbs, by including "field" before the list of household members, and by transposing "house" and "wife," thus separating the latter from the list and placing the charge against lust (i.e., coveting a neighbor's wife) on its own line.
Nevertheless, it is evident that, even with these alterations, Deuteronomy itself treats its Decalogue as a reiteration of the very "Ten Words" spoken by God out of the midst of the fire at the mountain of God--namely, as an echo of Exod 20:1-17 (cf. Deut 5:4-5, 22 with 4:12-13 and 10:4). After the 40 years in the wilderness (1:3-4; 4:45-46), the Ten Words have been updated, probably for pastoral purposes. But Moses still stresses that, while the Deuteronomic version is a secondary account, it is nevertheless "just as Yahweh your God commanded you" (Deut 5:12, 16). As concluded by Norbert Lohfink, this formulaic back-reference ensures that "in spite of the changes and additions that have been made [in the Deuteronomic version], at bottom nothing is commanded that is not also in the older version." When we think of the Ten Commandments, we should think of the lists in Exod 20:1-17 and Deut 5:6-21.
 For a helpful, balanced, and scripturally faithful assessment of the role of the Decalogue in ancient Israel as witnessed to in the Old Testament, see Daniel I. Block, "Reading the Decalogue Right to Left: The Ten Principles of Covenant Relationship in the Hebrew Bible," in idem, How I Love Your Torah, O LORD! Studies in the Book of Deuteronomy (Eugene, OR: Cascade, 2011) 21-55. Block argues that, more than the Decalogue, Deuteronomy as a whole played the most foundational role in shaping a Yahwistic worldview and the Scriptures.
 This view is espoused most recently by David H. Aaron, Etched in Stone: The Emergence of the Decalogue (New York: T & T Clark, 2006). Other scholars have posited a "curse Decalogue" in Deut 27:15-26, but the curses number twelve, not ten, and they are never associated with the Decalogue.
 For helpful reflections on this possibility, see Daniel I. Block, "'You Shall Not Covet Your Neighbor's Wife': A Study in Deuteronomy's Domestic Ideology," JETS 53 (2010) 449-74 (repr. idem, in The Gospel according to Moses: Theological and Ethical Reflections on the Book of Deuteronomy [Eugene, OR: Cascade, 2012] 137-68).
 Norbert Lohfink, "The Decalogue in Deuteronomy 5," in Theology of the Pentateuch: Themes of the Priestly Narrative and Deuteronomy (trans. Linda M. Maloney; Minneapolis: Fortress, 1994) 262. While I agree with Lohfink on this point, I do not agree with his historical conclusions or with his assertion that the Sabbath is "the principal commandment" of the Deuteronomic Decalogue.