I notice that many English translations render διαθήκη in Hebrews 9:16 as either "will" or "testament":
Is that correct or should it just be rendered "covenant" in all places where it appears in the NT?
I understand why it is often rendered "will" or "testament" in English translations but I believe this is an error and διαθήκη should be translated "covenant" (or "agreement" if that is what the OT has) in all places where it appears in the NT.
For reference I supply the entry from BDAG:
διαθήκη, ης, ἡ (Democr., Aristoph.+; ins, pap, LXX, En, TestSol, TestAbr, Test12Patr; ParJer 6:21; ApcEsdr, ApcMos; AssMos Fgm. a; Philo, Joseph., Just.; Mel., HE 4, 26, 14) apart from the simplex θήκη ‘case, chest’, for the mng. of this word one must begin with the mid. form of the verb διατίθεμαι, which is freq. used in legal and commercial discourse of disposition of things (s. L-S-J-M s.v. διατιθημι B), w. implication of promissory obligation. Disposition of one’s personal effects would naturally come under testamentary law, hence ① last will and testament (so exclusively in Hellenistic times, Eger [s. 3 below] 99 note; exx. e.g. in Riggenbach 292ff; Behm 10, 1; 2; Philo, Joseph., Test12Patr; loanw. in rabb.) Hb 9:16f; δ. κεκυρωμένη a will that has been ratified Gal 3:15; cp. 17, where δ. shades into mng. 2 (s. κυρόω 1, προκυρόω); s. also EBammel, below, and JSwetnam, CBQ 27, ’65, 373–90. On Jewish perspective s. RKatzoff, An Interpretation of PYadin 19—A Jewish Gift after Death: ProcXXCongPap 562–65. ② As a transl. of בְּרִית in LXX δ. retains the component of legal disposition of personal goods while omitting that of the anticipated death of a testator. A Hellenistic reader would experience no confusion, for it was a foregone conclusion that gods were immortal. Hence a δ. decreed by God cannot require the death of the testator to make it operative. Nevertheless, another essential characteristic of a testament is retained, namely that it is the declaration of one person’s initiative, not the result of an agreement betw. two parties, like a compact or a contract. This is beyond doubt one of the main reasons why the LXX rendered בְּרִית by δ. In the ‘covenants’ of God, it was God alone who set the conditions; hence covenant (s. OED s.v. ‘covenant’ sb. 7) can be used to trans. δ. only when this is kept in mind. So δ. acquires a mng. in LXX which cannot be paralleled w. certainty in extra-Biblical sources, namely ‘decree’, ‘declaration of purpose’, ‘set of regulations’, etc. Our lit., which is very strongly influenced by LXX in this area, seems as a rule to have understood the word in these senses (JHughes, NovT 21, ’79, 27–96 [also Hb 9:16–20; Gal 3:15–17]). God has issued a declaration of his purpose Ro 11:27 (Is 59:21); 1 Cl 15:4 (Ps 77:37); 35:7 (Ps 49:16), which God bears in mind (cp. Ps 104:8f; 105:45 al.) Lk 1:72; it goes back to ancestral days Ac 3:25 (PsSol 9:10; ParJer 6:21). God also issued an ordinance (of circumcision) 7:8 (cp. Gen 17:10ff). Since God’s holy will was set forth on more than one occasion (Gen 6:18; 9:9ff; 15:18; 17:2ff; Ex 19:5 and oft.), one may speak of διαθῆκαι decrees, assurances (cp. διαθῆκαι πατέρων Wsd 18:22; 2 Macc 8:15.—But the pl. is also used for a single testament: Diog. L. 4, 44; 5, 16. In quoting or referring to Theophr. sometimes the sing. [Diog. L. 5, 52; 56] is used, sometimes the pl. [5, 51; 57]) Ro 9:4; Eph 2:12. Much emphasis is laid on the δ. καινή, mentioned as early as Jer 38:31, which God planned for future disposition (Hb 8:8–10; 10:16). God’s decree or covenant directed toward the Christians is a καινὴ δ. (δ. δευτέρα Orig., C. Cels. 2, 75) Lk 22:20; 1 Cor 11:25; 2 Cor 3:6; Hb 8:8; 9:15a; PtK 2 p. 15, 5, or δ. νέα Hb 12:24; PtK 2 p. 15, 6 which, as a δ. αἰώνιος (cp. Jer 39:40; En 99:2) Hb 13:20, far excels 7:22; 8:6 the παλαιὰ δ. 2 Cor 3:14, or πρώτη δ. Hb 9:15b, with which it is contrasted. Both are mentioned (Did., Gen. 46, 4; 235, 26) Gal 4:24; B 4:6ff (Ex 34:28; 31:18; Just., D. 67, 9). Blood was shed when the old covenant was proclaimed at Sinai Hb 9:20 (Ex 24:8); the same is true of the new covenant Hb 10:29. τὸ αἷμά μου τ. διαθήκης Mt 26:28; Mk 14:24 (ELohse, Märtyrer u. Gottesknecht2, ’63, 122–29) is prob. to be understood in connection w. this blood (s. WWrede, ZNW 1, 1900, 69–74; TRobinson, My Blood of the Covenant: KMarti Festschr. 1925, 232–37; for a critique of this view s. GWalther, Jesus, D. Passalamm des Neuen Bundes, ’50, 22–27 and JJeremias TLZ, ’51, 547. For Syriac background JEmerton, JTS 13, ’62, 111–17; s. also ÉDelebrecque, Études grecques sur l’vangile de Luc ’76, 109–21).—The v.l. Lk 22:29 may be derived from Jer 39:40 or Is 55:3 LXX (for the cognate acc. s. Aristoph., Aves 440).—δ. may also be transl. decree in the Ep. of Barnabas (4:6ff; 6:19; 9:6; 13:1, 6; 14:1ff δ. δοῦναί τινι); but the freq. occurrence of the idea of inheritance (6:19; 13:1, 6; 14:4f), makes it likely that the ‘decree’ is to be thought of as part of a will. ③ The mng. compact, contract seems firmly established for Gr-Rom. times (FNorton, A Lexicographical and Historical Study of Διαθήκη, Chicago 1908, 31ff; EBruck, D. Schenkung auf d. Todesfall im griech. u. röm. Recht I 1909, 115ff; JWackernagel, D. Kultur d. Gegenw. I 82 1907, 309). It remains doubtful whether this mng. has influenced our lit. here and there (exc. quite prob. Lk 22:29 v.l. with its administrative tenor; the phrase διατίθεμαι δ. as Aristoph., Av. 440 of a treaty agreement), but the usage of the term δ. in such sense would again serve as a bridge to LXX usage.—The expr. ἡ κιβωτὸς τ. διαθήκης covenant chest i.e. the sacred box (Eng. ‘ark’ as loanw. from Lat. arca) that symbolized God’s pledge of presence w. Israel (Ex 31:7; 39:14 al.) Hb 9:4; Rv 11:19 or αἱ πλάκες τ. διαθ. (Ex 34:28; Dt 9:9, 11) Hb 9:4 would have required some acquaintance with Israelite tradition on the part of ancient readers.—ERiggenbach, D. Begriff d. Διαθήκη im Hb: Theol. Stud. f. TZahn 1908, 289ff, Hb2 1922, 205ff al.; ACarr, Covenant or Testament?: Exp. 7th ser., 7, 1909, 347ff; JBehm, D. Begriff D. im NT 1912; ELohmeyer, Diatheke 1913; WFerguson, Legal Terms Common to the Macedonian Inscr. and the NT, 1913, 42–46 (testamentary exhibits); HKennedy, Exp. 8th ser., 10, 1915, 385ff; GVos, Hebrews, the Epistle of the Diatheke: PTR 13, 1915, 587–632; 14, 1916, 1–61; OEger, ZNW 18, 1918, 84–108; EBurton, ICC Gal 1921, 496–505; LdaFonseca, Διαθήκη foedus an testamentum?: Biblica 8, 1927; 9, 1928; EBammel, Gottes διαθήκη (Gal 3:15–17) u. d. jüd. Rechtsdenken, NTS 6, ’60, 313–19; NDow, A Select Bibliography on the Concept of Covenant, Austin Seminary Bulletin 78, 6, ’63; CRoetzel, Biblica 51, ’70, 377–90 (Ro 9:4); p 229 DMcCarthy, Berit and Covenant (Deut.), ’72, 65–85; EChristiansen, The Covenant in Judaism and Paul ’95.—DELG s.v. θήκη. M-M. TW. Sv. Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., pp. 228–229). Chicago: University of Chicago Press.
I should also point out that διαθήκη is used throughout the LXX to render "brit" or "covenant" from the Hebrew.