There is a Greek word at the center of how Christians describe their Bible. We call the two main divisions of the Bible the Old Testament and the New Testament. The Greek word behind both names is diathēkē.
Most Christians have used this language their entire lives without thinking about what it means. The “Old Testament” is not just “the older books.” It is the older testament. The “New Testament” is not just “the more recent books.” It is the newer testament. And the question of what a testament is — and what makes one new — is what this entry is about.
It is also the word Jesus used the night before He died, when He took the cup at the Last Supper and said: “this is my blood, the blood of the covenant” (Matt 26:28, NET). Or, as Paul preserves the saying: “This cup is the new covenant in my blood” (1 Cor 11:25, NET). The Greek word behind “covenant” in those words of institution is diathēkē — the same word that gives your Bible its name.
This is the entry where those two uses are shown to be the same word doing the same theological work.
The Word
διαθήκη (diathēkē), pronounced dee-ah-THAY-kay. A feminine noun, first declension. From the verb diatithēmi (διατίθημι), “to arrange, to set in order, to dispose.” The basic sense of diathēkē is what someone has set in place — an arrangement, an ordered disposition, especially one with binding force.
In ordinary Greek usage of the New Testament era, diathēkē typically meant a last will and testament. When a Greek-speaking person of the first century heard diathēkē, the legal sense was the default: an instrument by which a person sets out how his property will be distributed after his death. The Greek word for an agreement between parties — what English-speakers usually mean by “covenant” — was synthēkē, not diathēkē. Synthēkē names a mutual contract negotiated between roughly equal parties; diathēkē names a one-sided arrangement laid down by one party for the benefit of others.
So when the Septuagint translators needed a Greek word for the Hebrew berit — the word for God’s covenants with His people — they chose diathēkē rather than synthēkē. The choice was theological. God’s covenant with Israel was not a contract between equals. It was God’s sovereign disposition, established on His own initiative, on terms He set, taking effect by His own action. Diathēkē captured that asymmetry in a way synthēkē could not. The Greek-speaking Jewish translators heard the divine covenants as more like a sovereign last will than a mutual agreement, and they passed that hearing on to the New Testament writers.
The New Testament inherits this. When the NT writers speak of diathēkē, they typically mean covenant in the berit sense — but they preserve the Greek connotations of unilateral disposition, of testamentary arrangement, of inheritance bequeathed rather than earned. And in some passages, especially in Hebrews, they exploit the dual sense deliberately.
Range of Meaning
In its New Testament usage, diathēkē covers:
- A covenant — a binding arrangement established by God with His people. The dominant sense, drawing on the Hebrew berit tradition.
- A last will and testament — the legal instrument by which a person disposes of property to take effect at death. Especially clear in Hebrews 9:15–17.
- A unilateral divine disposition — God’s sovereign arrangement of salvation, on His own initiative, for the benefit of those He calls.
- The Old and New Testaments as salvation-historical realities — the covenants God established with Israel under Moses, and the covenant established in Christ.
The two senses — covenant and testament — are not really separable in the New Testament’s use. They are the same word doing both jobs. When Christ speaks of the “the new covenant in my blood.” He is announcing both a covenant established by God’s action and a testament that takes effect through His own death. The English translation often has to choose; the Greek does not.
Where You’ll Meet It
“for this is my blood, the blood of the covenant, that is poured out for many for the forgiveness of sins.” (Matthew 26:28, NET)
The Words of Institution, in Matthew’s version. The diathēkē is being established in Christ’s blood. Worth noting: this is the new diathēkē (Luke and Paul add the word “new” explicitly), but it is also continuous with the old — Christ is fulfilling, not abolishing, the covenantal arc that runs from Abraham through Moses.
“And in the same way he took the cup after they had eaten, saying, ‘This cup that is poured out for you is the new covenant in my blood.’” (Luke 22:20, NET)
Luke’s version. The explicit word “new” appears. The new diathēkē is grounded “in” Christ’s blood — established by, given through, made effective by His death.
“In the same way, he also took the cup after supper, saying, ‘This cup is the new covenant in my blood. Do this, every time you drink it, in remembrance of me.’” (1 Corinthians 11:25, NET)
Paul’s preservation of the same saying. The Lord’s Supper is the meal of the new diathēkē. We will come to the “in remembrance of me” — anamnēsis — in Anamnēsis. The point for this entry is just that the Supper is the place where Christians receive the new testament’s gifts.
“And so he is the mediator of a new covenant, so that those who are called may receive the eternal inheritance he has promised, since he died to set them free from the violations committed under the first covenant. For where there is a will, the death of the one who made it must be proven. For a will takes effect only at death, since it carries no force while the one who made it is alive.” (Hebrews 9:15–17, NET)
This is the passage where the dual sense of diathēkē becomes most theologically explicit. The author of Hebrews uses diathēkē twice, and in the second occurrence it clearly means “will” or “testament” in the legal sense — a will takes effect at the death of the one who made it. The “new covenant” Christ mediates is also a “last will” that has now taken effect through His death. Both senses are operating in the same passage. The English translation has to keep both in view; some translations explicitly switch from “covenant” to “will” within these three verses to capture the play, while others note the dual sense in a footnote.
“Brothers and sisters, I offer an example from everyday life: When a covenant has been ratified, even though it is only a human contract, no one can set it aside or add anything to it. Now the promises were spoken to Abraham and to his descendant. Scripture does not say, ‘and to the descendants,’ referring to many, but ‘and to your descendant,’ referring to one, who is Christ. What I am saying is this: The law that came 430 years later does not cancel a covenant previously ratified by God, so as to invalidate the promise.” (Galatians 3:15–17, NET)
Paul argues from the diathēkē with Abraham — a covenant of promise, established with Abraham four centuries before the Mosaic law — to show that the law did not annul the promise. The Abrahamic diathēkē and the Mosaic diathēkē are not at war. The promise comes first; the law serves it; both are fulfilled in Christ.
“who made us adequate to be servants of a new covenant not based on the letter but on the Spirit, for the letter kills, but the Spirit gives life.” (2 Corinthians 3:6, NET)
Paul on apostolic ministry. The new diathēkē has ministers who serve it. The contrast with the “letter” (the law as condemnation) versus the “Spirit” (the gospel that gives life) connects directly to the law/gospel distinction we treated in Nomos.
“who are Israelites. To them belong the adoption as sons, the glory, the covenants, the giving of the law, the temple worship, and the promises.” (Romans 9:4, NET)
Paul listing Israel’s privileges. The covenants — plural — belong to Israel. The Christian church does not replace Israel as the recipient of the covenants; we are grafted into the covenants God made with Israel through faith in Christ, the Jewish Messiah (Rom 11:17–24).
What Confessional Lutherans Hear
For confessional Lutheran ears, diathēkē carries two emphases that much of the broader church lets fade.
First, diathēkē is fundamentally a gift, not a contract. The Greek choice of diathēkē rather than synthēkē for the divine covenants tells us how the Septuagint translators — and after them the New Testament writers — heard God’s relationship with His people. God is not negotiating a mutual agreement with creatures who could have terms to offer Him. God is sovereignly disposing of His own purposes for the benefit of those He has chosen to bless. The covenant is His; the terms are His; the binding force is His; the gifts it delivers are His. We receive the diathēkē. We do not bargain it.
This is one of the places confessional Lutheran theology pushes back, gently, against some forms of Reformed covenant theology that frame the covenant of grace in terms that can feel more bilateral than the New Testament’s diathēkē will support. The classical Reformed framework speaks of a “covenant of works” with Adam (which Adam broke) and a “covenant of grace” with Christ (in which the elect are included). Lutherans share with the Reformed the conviction that salvation has always been by grace through faith on the basis of Christ’s work. But Lutherans typically do not use the covenant-of-works / covenant-of-grace framework systematically. We tend to frame salvation history through the law/gospel distinction (Nomos) rather than through a covenantal scheme — partly because the diathēkē in Scripture is so consistently presented as God’s unilateral gift rather than as an agreement that requires our part to be valid.
The pastoral edge of this difference shows up in how the two traditions tend to talk about Christian assurance. Reformed treatments sometimes ask whether someone has truly met the conditions of the covenant. Lutheran treatments more typically direct the questioner to the unconditional promise of the testament — the gift God has given in Christ, sealed in baptism, received in the Supper. The covenant is His. The gifts are mine because He has put them in His will.
Second, there is one promise running through both testaments, and Christ is its substance. The “Old Testament” and the “New Testament” are not two separate religions. They are two phases of the same salvation history, both centered on Christ — the One promised in the Old, the One delivered in the New. The relationship between the testaments is not contradiction but fulfillment. The new does not erase the old; the new is what the old was always pointing toward.
This sets up a sharp pushback against several alternative readings.
Against dispensational theology — which divides salvation history into multiple covenants administered to different peoples in different ways, and which has sometimes taught that the law was the basis of salvation in the Old Testament era and grace is the basis in the New — Lutherans hold that salvation has always been by grace through faith on the basis of Christ’s work. Abraham believed God and it was counted to him as righteousness (Gen 15:6), and Paul uses Abraham specifically to teach that justification has always worked the same way (Rom 4:1–8, Gal 3:6–9). Old Testament saints were saved by faith in the coming Christ, the same way New Testament saints are saved by faith in the Christ who has come. The dispensational division of salvation history into eras with different terms misreads both testaments.
Against any “replacement theology” that treats the church as having replaced Israel — Lutherans hold that the church is grafted into the promises God made to Israel (Rom 11:17–24). The covenants belong to Israel (Rom 9:4); Gentile Christians are brought in through Christ, not in place of Israel. Paul’s careful treatment in Romans 9–11 of God’s ongoing faithfulness to Israel is not optional theology for confessional Lutherans.
Against Marcionism — the ancient heresy that rejected the Old Testament as the record of a different, lesser God — Lutherans hold that the God of the Old Testament is the God of the New Testament, and that the Old Testament is Christian Scripture. The Lutheran lectionary reads from the Old Testament every Sunday because the Old Testament is the church’s book. The Psalms are the church’s hymnal. The prophets are the church’s preachers. Christ is the substance of both testaments.
The pastoral payoff: when you read your Old Testament, you are reading the same story of grace that you read in your New Testament. The God who delivered Israel from Egypt is the God who delivered you from sin. The promises God made to Abraham are promises that come to you through Christ. The Passover lamb is your Passover Lamb. The exile is your exile, and the return is your return. The Old Testament is not the optional warm-up to the New Testament; it is the same story Christ comes to fulfill, and the church reads it as her own.
Where People Get It Wrong
Three common ways.
The first is the dispensational division of salvation history. This view, popularized in English-speaking Christianity through the Scofield Reference Bible and various forms of dispensational premillennialism, divides God’s dealings with humanity into multiple covenantal eras with different terms of relationship. In its sharper forms it has held that the Mosaic covenant was a covenant of works (offering salvation through law-keeping for Israel under the old administration), while the New Covenant is a covenant of grace (offering salvation through faith for the church under the new administration). Confessional Lutherans, with the broader Reformation tradition, reject this. Salvation has always been by grace through faith. The covenants differ in administration; they do not differ in the way they save. Abraham, Moses, David, and the New Testament Christian are saved the same way — by faith in the promises of God that come to fulfillment in Christ.
The second is the over-systematized covenant scheme that sometimes appears in Reformed treatments. This is a milder error than dispensationalism and shares much of the Lutheran view on substance. The concern is more about emphasis: when the covenant becomes a systematic framework that orders all of theology, the gospel can begin to feel like an arrangement whose terms must be met rather than a testament whose gifts are simply received. Lutheran theology has typically preferred the law/gospel framework — which keeps the unconditional gift-character of the gospel in the foreground — over the covenantal framework. The substance is similar between the two Reformation traditions; the framing is different.
The third is the Marcionite drift — treating the Old Testament as discarded or as belonging to a fundamentally different religion. This drift can appear in surprising places: in Christians who avoid the Old Testament because they find it dark or unfamiliar, in churches whose preaching never engages the Old Testament narrative, in popular teaching that frames the God of the Old Testament as harsh and the God of the New Testament as loving. The Lutheran response is to read the Old Testament as Christian Scripture, to preach Christ from the Old Testament, and to teach the unity of the diathēkē across both testaments. The God of Sinai is the God of Calvary. The God who fed Israel with manna is the God who feeds the church with the body and blood of His Son. The same God. The same grace. The same testament, now made new in Christ.
So What
When you read the Bible, read all of it as Christian Scripture. The Old Testament is not the optional historical background to the New Testament. It is the first half of the story Christ comes to fulfill, and the church receives it as her own. The Psalms are her prayer book. The prophets are her preachers. The Pentateuch is her foundation. The historical books are her family history.
When you receive the Lord’s Supper, you receive the new diathēkē in Christ’s blood. The Supper is the meal of the testament. Christ is the testator. The gifts He bequeathed take effect through His death, which has now occurred. As often as you drink it, you receive what He left you in His will — the forgiveness of sins, the body and blood, the foretaste of the feast to come. For the sacramental dimensions of this, see Anamnesis, Soma, and Haima. For now, the point is simply that the words of institution are testamentary words. Christ is naming what He leaves to those who believe.
For Sunday school teachers and Bible study leaders: when you teach an Old Testament passage, look for the gospel shape that runs through it. The promise to Abraham. The deliverance from Egypt. The Passover sacrifice. The exile and the return. All of these are gospel-shaped because they are all parts of the one diathēkē that comes to fulfillment in Christ. Teaching the Old Testament as Christian Scripture is one of the most important services a lay teacher can render to a congregation that has often been trained to skip it.
The testament connects the diagnosis of sin and the need for a Savior with the announcement of what the Savior has done. The diathēkē is the doorway. The gospel walks through it.