The Unalterable Promise Text: Galatians 3:15-18
Introduction: God's Sealed Will
We live in a world of fine print, shifting terms, and broken contracts. Men make promises and then hire lawyers to find ways out of them. Agreements are made with fingers crossed. But when we come to the Scriptures, we are entering an entirely different reality. We are dealing with a God who does not lie, whose Word is His bond, and whose covenants are more stable than the foundations of the earth. In our passage today, the apostle Paul is driving this point home with the force of a sledgehammer, striking at the very root of the Judaizers' error.
The Galatians were being tempted to abandon the sheer, unadulterated grace of the gospel for a system of religious performance. The Judaizers had come among them, not to add a missing ingredient to their faith, but to poison the well entirely. Their message was this: "Yes, Jesus is fine, but you also need the works of the Law. You need circumcision. You need to get on the right side of Moses to make sure you are truly on the right side of God." Paul's response is not one of gentle correction; it is a thunderous rebuke. He is not tweaking their theology; he is rescuing them from a different gospel, which is no gospel at all.
To make his case, Paul appeals to the bedrock of God's dealings with humanity: the covenant of promise made to Abraham. This was the foundational charter of salvation. The Judaizers were trying to attach a legal amendment, a rider clause, to this foundational promise. They were treating the Law given to Moses as an update that modified the original terms. Paul argues that this is not only theologically bankrupt, but it is also a violation of basic common sense. He uses an illustration from everyday human affairs to show that God's covenant is far more secure than any human will or testament. If even men know how to make a binding agreement, how much more the living God?
What is at stake here is everything. Is our salvation a gift, received by faith alone, based on an unbreakable promise? Or is it a wage, earned by our performance, subject to our ability to keep the rules? Paul is showing us that God established the principle of grace and promise hundreds of years before the Law ever showed up. The Law came later, not to change the deal, but for an entirely different purpose. To misunderstand this is to misunderstand the entire flow of redemptive history and to trade the glorious liberty of the sons of God for the miserable bondage of slaves.
The Text
Brothers, I speak in human terms: even though it is only a man’s covenant, yet when it has been ratified, no one sets it aside or adds conditions to it. Now the promises were spoken to Abraham and to his seed. He does not say, “And to seeds,” as referring to many, but rather to one, “And TO YOUR SEED,” that is, Christ. And what I am saying is this: the Law, which came 430 years later, does not invalidate a covenant previously ratified by God, so as to abolish the promise. For if the inheritance is by law, it is no longer by promise, but God has granted it to Abraham through promise.
(Galatians 3:15-18 LSB)
The Ratified Covenant (v. 15)
Paul begins with an appeal to common sense, drawing an analogy from human legal practice.
"Brothers, I speak in human terms: even though it is only a man’s covenant, yet when it has been ratified, no one sets it aside or adds conditions to it." (Galatians 3:15)
Paul addresses them as "brothers," a pastoral touch in a letter filled with sharp rebuke. He is about to lay down some heavy theology, so he brings it down to a level anyone can grasp: "I speak in human terms." He is saying, "Let's just think about how things work in the ordinary world." The word for "covenant" here can also be translated as a "will" or a "testament." Think of a last will and testament. Once it is legally executed, once it is ratified, it is fixed. You cannot just come along later and scribble in new conditions. You cannot add a clause that says, "The inheritance is yours, provided you now mow the lawn for the next twenty years." The terms are set. The deal is done.
This is an argument from the lesser to the greater. If this is how honorable men conduct their affairs, how much more steadfast is the God of all creation? If a human promise, once formalized, is considered binding, then what shall we say of a covenant ratified by God Himself? The Judaizers were treating God's covenant with Abraham as though it were a rough draft. They were acting as though the Law of Moses was God's editorial correction, adding a host of new conditions that Abraham knew nothing about. Paul says this is legal nonsense. God's covenant with Abraham was a done deal. It was ratified. It cannot be altered or amended by a later addition.
The Singular Seed (v. 16)
Next, Paul makes a precise, grammatical point that is absolutely central to his argument. He identifies the true recipient of the promise.
"Now the promises were spoken to Abraham and to his seed. He does not say, 'And to seeds,' as referring to many, but rather to one, 'And TO YOUR SEED,' that is, Christ." (Galatians 3:16)
This is a laser-guided missile into the heart of the Judaizers' system. They believed that the promise was to the physical descendants of Abraham, the Jewish people as a collective, and that therefore, keeping the Jewish law was the way to remain in the covenant. But Paul, under the inspiration of the Holy Spirit, points to the specific wording of the Genesis promise. The promise was made to Abraham and his "seed" (sperma in the Greek), which is a singular noun. He says God did not say "seeds," in the plural, as though the promise was distributed among countless individual heirs running in a thousand different directions. The promise was directed to one, specific, singular Seed.
And then he drops the bombshell: "that is, Christ." Jesus Christ is the Seed of Abraham. He is the true heir, the focal point of the entire promise. All the promises of God find their "Yes" in Him (2 Cor. 1:20). This means that the way to become a child of Abraham and an heir of the promise is not by being born a Jew, or by getting circumcised, or by keeping kosher. The only way to be an heir is to be united to the Heir. If you are in Christ, you are a child of Abraham. If you are not in Christ, you are not, regardless of your bloodline. This demolishes any notion of ethnic privilege. The inheritance does not flow through DNA; it flows through faith in the one true Seed, Jesus Christ.
Promise Trumps Law (v. 17)
Paul now brings his legal and theological points together, using simple arithmetic to show the priority of the promise.
"And what I am saying is this: the Law, which came 430 years later, does not invalidate a covenant previously ratified by God, so as to abolish the promise." (Galatians 3:17)
Here is the core of the argument. The timeline is crucial. The promise was given to Abraham. Then, 430 years passed. After four centuries of the promise being the only game in town, God gave the Law through Moses at Mount Sinai. Paul's logic is irrefutable: a legal instrument that comes into effect 430 years after a covenant has been ratified cannot possibly nullify that original covenant. You cannot retroactively change the terms of a sealed agreement.
The Law, therefore, was never intended to be a new way of salvation. It did not supersede, invalidate, or abolish the promise. The promise of grace through faith remained the operating principle. God did not put His people on a grace plan for 430 years and then suddenly switch them to a works-based performance plan. The covenant of promise is the permanent, unshakable foundation. The Law was an addition, yes, but it was not an amendment. It was added for a different reason entirely, which Paul will explain later, namely, to reveal sin and to act as a guardian to drive us to Christ. The Judaizers had it completely backwards. They were trying to make the foundation subservient to the temporary scaffolding.
An Unmixable Inheritance (v. 18)
Finally, Paul lays out the two principles as mutually exclusive. You have to choose one or the other. You cannot have it both ways.
"For if the inheritance is by law, it is no longer by promise, but God has granted it to Abraham through promise." (Galatians 3:18)
This is the logical conclusion. Paul presents two opposing principles for receiving the inheritance: law or promise. If the inheritance comes through law, it means you earn it. It is a wage you receive for your work. If it comes through promise, it means you receive it as a free gift. It is pure grace. These two things are like oil and water; they do not mix.
You cannot have an inheritance that is 90% gift and 10% wages. The moment you introduce the principle of law-keeping as the basis for your standing with God, you have nullified the principle of promise. If you have to work for it, it is not a gift. Paul is forcing the Galatians to see that the Judaizers' teaching was not a friendly supplement to the gospel, but a hostile takeover. It was a complete system change. It was a move from the category of "gift" to the category of "debt."
But Paul concludes by returning to the historical fact. How did Abraham get the inheritance? "God has granted it to Abraham through promise." The verb "granted" is one of pure grace. Abraham did not earn it, achieve it, or deserve it. God freely gave it. That is the charter of our faith. That is the bedrock reality. The entire plan of salvation, from start to finish, is a matter of God's gracious promise, received by simple faith.
Conclusion: Standing on the Promise
The implications of this are immense. We are not saved by a mixture of grace and works. We are not saved by Christ-plus-our-efforts. We are saved by grace alone, through faith alone, in Christ alone. The promise made to Abraham finds its complete fulfillment in the person and work of Jesus Christ, the singular Seed.
When you came to Christ, you did not sign up for a probationary period where your performance would determine your final standing. You were written into God's will. You were united to the Heir. You became a recipient of a promise that was ratified by God centuries before the Law was ever given. That promise was sealed not with ink, but with the very blood of the Testator, Jesus Christ Himself.
Therefore, any voice, whether from outside the church or from within, that tells you to look to your own performance, your own obedience, or your own spiritual discipline as the basis for your acceptance with God is a lying voice. It is the voice of the Judaizers. Our obedience and good works are the fruit of our salvation, not the root of it. They are our thankful response to the gift, not a down payment on it.
We stand on an unalterable promise. We are heirs according to a covenant that cannot be annulled. Our inheritance was not earned by us, but it was granted by God's grace to Abraham, and secured for us by the one true Seed, Jesus Christ. Do not let anyone tamper with God's will. Do not let anyone add conditions to His promise. Rest in the finished work of Christ, the sole heir of all things, in whom you are a fellow heir of the grace of life.