I received this question a while back, and it turns out to be a fascinating exercise in exegesis. In fact, after reading the literature, I had to call my good friend George Guthrie, a Hebrew’s specialist, to make sure I was understanding the issue.
The fun part is that the Greek is, shall we say, “atrocious,” “convoluted,” or perhaps I should say, “not quite clear.” As a result, it becomes an exercise in exegetical method. So hold onto your hats.
διαθήκη has two basic meanings in BDAG. It is the technical term for a last will and testament. It is also the NT translation of the Hebrew בְּרִית, “covenant.”