WILL (TESTAMENT) (διαθήκη, G1347, disposition, ordinance, full arrangement). The published will of a person (in the NT, God or Christ) concerning his possessions and his heirs.
In the LXX the term διαθήκη, G1347, is the common tr. for the Heb. word בְּרִית, H1382, covenant. This suggests that the Gr. trs. understood the OT as more than simply an agreement between parties. Rather, it was a publication of God’s sovereign will for man’s salvation in which man cooperated.
By NT times the primary meaning of διαθήκη, G1347, as will or testament had so far exerted itself that the idea of agreement is largely lacking. Particularly in non-Biblical Gr. lit., the term connoted simply “last will and testament.” Paul uses the word in this light (Gal 3:15-17, KJV covenant) in demonstrating that God’s covenant, like a man’s will, can be changed only by the originator. Similarly, the writer to the Hebrews expresses the terms of the covenant (διαθήκη, G1347) on an analogy with a will (διαθήκη, G1347) (9:15-18). He shows that when the testator dies, then only does the will become operative. Thus, since Christ has died, all the promises of the covenant are available to man.
Generally, however, the NT uses the term in a broader sense more in keeping with the OT idea. Jesus’ statement of a “new testament” in His blood tells of a new disposition of affairs for the redemption of men.
G. Kittel, TWNT, II, 124-134.