Ordinarily a 'testament' is a declaration of a person's intention (hence the verb 'testify'), and is often used of a person's last will, testifying to what he/she intends to do with his/her estates. The use of the term in denoting the two parts of the Bible as the Old and the New Testaments has a biblical basis but is also the result of the quirkiness of translation.
In reproving the Galatian Christians for their swiftness in "turning back to those weak and miserable principles" and being "enslaved by them all over again," Paul speaks of Sarah and Hagar as the figures of the "two covenants" (4:24). The Greek word for 'covenant' here is diatheke. In non-biblical Greek, diatheke almost always denotes a last will or testament. In the Septuagint, however, the Hebrew word for covenant, berith, is consistently translated as diatheke. Diatheke is, notes J. B. Lightfoot, "as universally used of a covenant . . . Nor in the New Testament is it ever found in any other sense, with one exception. Even in this exceptional case, Heb. ix. 15-17, the sacred writer starts from the sense of a 'covenant', and glides into that of a 'testament.'" Thus, it was in this somewhat soggy bog of translational history that the two parts of our Bible came to be called in English "the Old and New Testaments." Translations of the Bible in other languages happily do not provide such a source for debate. The Chinese designation, e.g., is simply "the older part or the newer part of Scriptures," while the Bahasa Malaysia (Malay) Bible has simply "the Older Promises and the New Promises" (Perjanjian Lama and the Perjanjian Baru). But, in that a testament is intended to testify to the author's intention, the term 'testament' to denote the two parts of the English Bible, nonetheless, lives up superbly to its proper role, even if the word is otherwise and usually used with a slightly different connotation.
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