The word “testament,” as in OT or NT, is a translation of the Latin word testamentum, which in turn was a translation of the biblical idea of “covenant.” As many have noted, it is an inadequate translation of the biblical term used for covenant diathēkē. The Latin word testamentum indicates “a last will and testament,” and English “testament” follows suit. But the Old and New Testaments should be named the Old and New “Covenants.” This terminology would indicate a dynamic living relationship/agreement between God and his people—not a “last will and testament” by God to his people.
The Septuagint (LXX) (see below, Ancient Versions, pp. 121ff.), a Greek translation of the Hebrew Bible, used the Greek term, diathēkē to translate the Hebrew berîṯ, “covenant,” rather than sunthēkē. This latter Greek term indicated a covenant between equals; the former referred to a covenant or testament between unequals. To the biblical writer, the parties in the covenant between Israel and God were not equal. And, just as importantly, the word diathēkē, though it could mean testament as in “last will and testament,” could also mean covenant. It is unfortunate that English translators have used testament, for, as far as English is concerned, “covenant” is much more suitable than testament for translating diathēkē. This Greek term catches well the meaning of the Hebrew, which implied the “cutting of a covenant” between the relevant parties.
So “testament” is not a helpful English translation to the extent that it misses the biblical sense of a living “covenant” between two persons or parties. Moreover, calling the Hebrew Scriptures the “Old” Testament is open to abuse, since there are several key covenants in the OT, such as the Abrahamic Covenant, Sinai Covenant (the one most referred to), Davidic Covenant, Palestinian Covenant, and the New Covenant (Jer 31:31-34). In an essential way many would hold that all the these “old” covenants are still vital and being realized to some extent. Jesus came not to destroy the law, but to “fulfill” the law; and, in the case of the New Covenant of Jeremiah, he came to institute that covenant (Mk 12:24; Mt 26:28; Lk 22:20; 1Co 11:23-25). Hence, Jeremiah's New Covenant is a reaffirmation and realization of the “Old” Covenant in Christ through/by the Holy Spirit, while in reality the whole old covenant (entire corpus of Scripture) is an announcement and preparation for the New Covenant.
The writings we call the Old Covenant comprise over three-fourths of the Christian Bible. The early church fathers, Tertullian (a.d. 160-230) and Origen (c. a.d. 185-254), were the first to designate the collected writings of the Jewish Scriptures taken over by the church as the “Old Testament.” (Tertullian used the word instrumentum, not testamentum.)
In the NT, Paul refers to the reading of the “old covenant” (2Co 3:14, palaias diathēkēs) as equal to “when Moses is read” (2Co 3:15). The writer of Hebrews speaks of the Mosaic Covenant as “that first covenant” (Heb 8:7) and the New Covenant in Christ as a “second covenant” (8:7), but then also describes the first covenant as having grown old (8:13, pepalaiōken tēn prōtēn; cf. 9:15). It is clear that the entire Old Covenant as we understand it is not referred to in these passages. It is unfortunate that the entire corpus of texts comprising our present Old Covenant came to be called “the Old Testament [Covenant]”; since it contains, among many things, the announcement promising the “New Testament [Covenant]” as well (e.g., Jer 31:31-34; Heb 8:7-13), which was to complete, actualize, and expand the heart of the covenants in the OT itself, but, at the same time, tie the two covenants together organically. Jesus came not to abolish the essential religious, moral, and ethical content and heart of the Mosaic Covenant, but to bring its lofty, moral, spiritual, and ethical goals representing the will of the covenant God, Yahweh, to realization (Mt 5:17).
It was the nature and character of the ancient Hebrew Scriptures, which are identical to the Protestant form of the OT canon, that caused them to become an integral part of the Christian Bible. The early Christians appealed to the Hebrew Scriptures to explain what had happened, was happening, and would happen among them; they also used it to buttress and establish their own faith. They were convinced that the man Jesus was none other than the Messiah (Messiah, OT = Christ, NT) of the ancient Hebrew Scriptures. They believed that the kingdom of God depicted in them was dawning in Jesus' life, words, and actions, and in and among those disciples and followers he was calling and gathering around him. The new “people of God” were the product of the New Covenant now realized, but also a continuation of the “people of God” depicted in the Old Covenant (Ro 2:28-29), which was now seeing its true fruition and completion in the New Covenant.
Both A. M. Hunter and F. F. Bruce (Bruce, Books and Parchments, 73-75) note at least three themes that tie the Old and New Covenants together: (1) God coming as Savior, bringer of salvation, ultimately through Christ; (2) redeemed community of God's people (ekklēsia used in both LXX and NT for this group), the elect people of God, (3) God's free electing love bestowed upon all who exercise faith in him. And finally, it can be said with certainty, that neither covenant is sufficiently comprehended without the other. The Old Covenant is the launching pad for the ongoing fulfillment found within its own pages and in the New Covenant itself.