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Reformed theology, as many have said, is covenant theology, for the concept of covenant has shaped the development of Reformed thinking. We should expect as much because of our doctrine of sola Scriptura, which says that the Bible is the only infallible authority for Christian faith and practice. Therefore, we want to structure all theological understanding according to Scripture. This demands covenant theology, since covenant is an organizing principle in Scripture.

Given the importance of the biblical doctrine of covenant, all Christians should have at least a basic understanding of what the Bible means by the term covenant. In the Old Testament, the Hebrew word we translate as “covenant” is berith, and it was used for centuries by the ancient Israelites. However, after Alexander the Great conquered the Eastern Mediterranean world and brought with him the Greek language, many Jews became more familiar with Greek than with Hebrew. Consequently, a Greek translation of the Hebrew Old Testament was made during the third century B.C. We know this translation as the Septuagint, and it had a tremendous impact on the writers of the Greek New Testament.

One challenge that the Jews who produced the Septuagint faced was the decision about how best to translate the Hebrew term berith or “covenant,” since there wasn’t any Greek word that precisely matched the Hebrew term. Eventually, the word chosen by the Septuagint translators was the Greek word diathēkē, and this was adopted by the New Testament writers, who, for the most part used diathēkē for the concept of “covenant.”

Some confusion arises here, however, because the Greeks used the word diathēkē in the sense of "testament." That's because a testament in Greek culture, at least at that time, included several nuances that made it di erent from the Hebrew concept of covenant. First, in Greek culture, a diathēkē—a testament—could be changed at any time by the testator while the testator was still alive. The testator could make up his last will and testament and then get irritated at his heirs and write them out of his will. And that continues to this day, for we know that people are sometimes disinherited, or written out of the will of a friend or family member. But that’s significantly different from a diathēkē—a testament or covenant—made by God. When God makes a covenant with His people, He can punish them for covenant breaking, but He never cancels the covenant promises He has made.

Another difference between the Greek understanding of diathēkē as a testament and the Hebrew understanding of berith as a covenant is that in the Greek world, the benefits of the testament, or diathēkē, did not accrue until after the testator died. Obviously, when God makes a covenant, His people don’t have to wait for Him to die to inherit the blessings of that covenant, because He’s incapable of dying.

Given those two great weaknesses, why did the Septuagint translators and the New Testament authors choose the Greek word diathēkē (“testament”) to translate the Hebrew berith,/ (“covenant”)? Because it was a better choice than the alternative Greek term—synthēkē. That word features the prefix syn-, which we see in such English terms as synonym, syncretism, and synchronization. It simply means “with.” And the idea of a synthēkē in Greek culture was an agreement between equal partners, an agreement with the consent of peers. The Jewish translators wanted to maintain that the covenants that God makes with His people are made between a superior and a subordinate, not between two equal parties. So, the word synthēkē was rejected.

The Septuagint translators and New Testament authors chose _diathēke_̄ because in its original use, before the Greeks came to use it to mean “testament,” it had reference to what is called “the disposition for one’s self.” A diathēkē had to do with an individual’s disposition of his goods orproperty for himself; that is, it referred to his sovereign determining of his heirs. To this day, that’s how we understand the concept of a testament or will. That aspect of disposition is an element that reflects the Hebrew concept because in His covenant, God sovereignly determines to give promises to whom He will give promises.

He made a covenant with Abraham, not Hammurabi. He chose the Jews, not the Philistines. He entered into a covenant relationship with them and said, ”I will be your God and you will be my people.” That’s a choice God made, not the Jews. So, even though the Greek word diathēkē includes aspects that don’t overlap with the Hebrew notion of a covenant, it carries the key notion of sovereign determination.

This might seem pedantic, but there’s one important point to take away from this discussion. If God’s covenant were a synthēkē made between two equal parties, all would be lost. Our role in maintaining the covenant relationship would be fully equal to His. But we are sinners who cannot keep covenant perfectly. A synthēkē would mean no salvation. A diathēkē is different. Because it is God’s sovereign administration with an unequal party, the onus for the fulfillment of the covenant's promises is on the greater party—the Lord Himself. He swore by Himself to uphold the covenant (Gen. 15; Heb. 6:13–20). His honor is on the line, and if He fails, His glory suffers. But we know that God cannot fail, that He will not surrender His glory (Isa. 48:11). In the Lord’s diathēkē, or covenant, He guarantees the fulfillment of His Word for His name’s sake, which means the redemption of His people is secure.