The Galatians were being foolish.
Or, more literally, they were not thinking things through.
The whole province was slipping into legalism, but they should have known better. In Galatians 3, Paul listed some of the theological reasons why the Galatians should not have been deserting Christ and falling for a false gospel.
One reason is due to the difference between a promise and a law:
And this I say, that the law, which was four hundred and thirty years later, cannot annul the covenant that was confirmed before by God in Christ, that it should make the promise of no effect. For if the inheritance is of the law, it is no longer of promise; but God gave it to Abraham by promise (Gal 3:17-18).
God made a promise to Abraham. Then God gave Israel a Law. The latter cannot annul the former. Why not? What’s the difference between a law and a promise?
Let’s begin with some similarities.
Both are legal. Both are covenants. So in that sense, a law and a promise are similarly binding.
Both involve works. Laws and promises are obligations to do something.
So what’s the difference between the two? You could say it is a difference in who is obliged to do the work. It is the difference in perspective between the first person and the second person, between “I” and “you.”
For example, if I make a promise to drive you, a friend, to the airport, who is doing the work? I am. I am promising to do the driving, to be there on time, and to get you there on time.
By contrast, if the State promulgates a law that says everyone must pay an 8% sales tax, who does that work? You, the taxpayers.
Who does the work in a promise? The promiser.
Who does the work in a law? Those under the law.
So how does this relate to Gal 3:17-18 and Paul’s argument that a later law cannot annul an earlier promise?
When God made a promise to Abraham (i.e., to justify the Gentiles by faith, vv 6-9), who does the work? God. He obligated Himself to keep it.
But when, 430 years later, God gave Israel the Law, who does the work? Israel. God obligated the Jews to keep it.
Here’s part of Paul’s point to the Galatians: Israel’s later obligation to keep the Law did not annul God’s earlier obligation to keep His promise to Abraham. Do you see why? A deal is a deal. If Israel had to keep the Law for God to fulfill His promise to Abraham, it would no longer be a promise. It would be something else. Hence, Paul says, “For if the inheritance is of the law, it is no longer of promise.
But God’s promise still stands. It was made to Abraham and Christ (v 16). The Mosaic Law didn’t—couldn’t—change it. The promise would stand. God would do it. He had to. It was His word. It was His obligation to fulfill His promise.
And if the Galatians had not been foolish—that is, if they had thought through the difference between a promise and a law—they would have stood on God’s promise, rather than fall for a false gospel.