Galatians 3:20 is literally translated:
A mediator is not of one, yet God is one.
The word “one” can mean either “one (as opposed to many)”; or it can mean “united (as opposed to divided)”. What does it mean in this verse? And what does this verse have to do with Paul’s argument about the law and covenants (Gal 3:15-19)?
(This post is part of a series)
Galatians 3:20 verse has spawned a multitude of interpretations, but a common thread in most interpretations is the juxtaposition of plurality and singularity. This is exacerbated by certain translations, which add a concept of plurality into the verse which isn’t there in the original (e.g. the ESV, “Now an intermediary implies more than one, but God is one”).
Wright, for example, understands Galatians 3:20 to mean that God, being one, desires one worldwide covenantal family demarcated by faith, rather than a plurality of different families. However, in normal Greek usage, the existence of a mediator (μεσίτης) usually implied a conflict or underlying disunity between two parties. Hence it seems that Paul’s argument is not about plurality but disunity between Israel and God.
This is backed up by the allusion to an important Old Testament verse. One of the foundational statements of the law was the Shema, with its tight indicative-imperative logic: “Hear, O Israel: The LORD our God, The LORD is One (κύριος ὁ θεὸς ἡμῶν κύριος εἷς ἐστιν). And you shall love the LORD your God with your whole heart, and with your whole soul, and with your whole strength” (Deut 6:4–5 LXX). The logic is that, because God is “one”, there must be “whole” undivided devotion to him.
But this is precisely what had not happened at the time of the giving of the law (Exod 32–34). God was about to destroy Israel for her outright apostasy with the Golden Calf, so a mediator (Moses) was introduced to the covenant, and God’s glory was veiled to Israel. The existence of a mediator proved that God and Israel were not united. Israel was never going to be able to fulfil the promise of international blessing. From her very inception, Israel failed to display the blameless walk required of the seed as a prerequisite for this covenant (Gen 17:1). So Paul adds a further argument to his proof that Christ, not Israel, is the true obedient seed of Abraham, not by means of a semantic trick (cf. 3:16) but here from the Torah itself.
 N. T. Wright, The Climax of the Covenant (London: T & T Clark, 1991) , 159.
 Wright, Climax, 168–72.
 Becker, NIDNTT 1:372–76; see also Craig R. Koester, Hebrews: A New Translation with Introduction and Commentary (The Anchor Bible; New York: Doubleday, 2001), 378–79.