What does the word διαθήκη (“covenant”) mean in Galatians 3:15?
(This post is part of a series)
Some interpreters understand the word to mean “last will and testament”. In this understanding, when Paul speaks in “human terms” (κατὰ ἄνθρωπον) about a “human” covenant (ἀνθρώπον [. . .] διαθήκην) he refers to the secular Graeco-Roman practice of will-making. According to this understanding, Paul then proceeds, by way of comparison, to show that just as a human will cannot be rejected (cf. ἀθετεῖ) or reordered (cf. ἐπιδιατάσσεται), so it is with God’s covenant.
Hughes, however, marshalling an impressive array of internal and external evidence, shows that διαθήκη in Gal 3:15 cannot possibly be used in the Hellenistic sense of “will”. Throughout the ancient world, a will could, and frequently was, nullified and changed by the testator. If Paul was using a will as his “human” example, the basic premise of his comparison would have been nonsense to his original readers. On the other hand, if Paul meant “covenant” according to our inductive definition (“elected relationship of obligation under oath”), the argument makes perfect sense. Sworn covenants between human beings in the Old Testament were inviolable (e.g. Josh 9:19–20, cf. 2 Sam 21:1–14). So, it seems, was the birds” covenant with Peisetaerus. Hence Paul is arguing from the general inviolability of covenants between human beings (3:15) to the inviolability of the particular covenant with Abraham (3:17). He is not introducing the idea of a “will” into his argument.
 E.g. Richard N. Longenecker, Galatians (Word Biblical Commentary 41; Dallas: Word, 1990); N. T. Wright, The Climax of the Covenant (London: T & T Clark, 1991) , 166; see also the Bible versions NJB, RSV, NRSV.
 John J. Hughes, “Hebrews IX 15ff. and Galatians III 15ff: A Study in Covenant Practice and Procedure”, Novum Testamentum 21 (1979): 27–96 (here 66–96).
 Of course, a “last will and testament” couldn’t be changed by anyone other than the testator, nor for this reason could it be changed after the testator’s death; but this is irrelevant, for Paul is claiming that God himself would not change his own previously ratified διαθήκη.
 Scott W. Hahn, “Covenant, Oath, and the Aqedah: Διαθήκη in Galatians 3:15–18”, Catholic Biblical Quarterly 67 (2005): 79–100 (esp. 83–86).
 Hahn, “Covenant, Oath, and the Aqedah”, 95.