Brothers and sisters, let me give you an example from everyday life. No one can get rid of an official agreement between people. No one can add to it. It can't be changed after it has been made. It is the same with God's covenant.
References for Galatians 3:15
The promises were given to Abraham. They were also given to his seed. Scripture does not say, "and to seeds." That means many people. It says, "and to your seed."(Genesis 12:7; 13:15; 24:7) That means one person. And that one person is Christ.
Here is what I mean. The law came 430 years after the promise. But the law does not get rid of God's covenant and promise. The covenant had already been made by God. So the law does not do away with the promise.
The great gift that God has for us does not depend on the law. If it did, it would no longer depend on a promise. But God gave it to Abraham as a free gift through a promise.
Then what was the purpose of the law? It was added because of human sin. And it was supposed to control us until the promised Seed had come. The law was put into effect through angels by a go-between.
A go-between does not take sides. God didn't use a go-between when he made his promise to Abraham. But the same God was at work in both the law and the promise.
So is the law opposed to God's promises? Certainly not! What if a law had been given that could give life? Then people could become right with God by obeying the law.
But Scripture announces that the whole world is a prisoner because of sin. It does so in order that what was promised might be given to those who believe. The promise comes through faith in Jesus Christ.
Before faith in Christ came, we were held prisoners by the law. We were locked up until faith was made known.
So the law was put in charge until Christ came. He came so that we might be made right with God by believing in Christ.
But now faith in Christ has come. So we are no longer under the control of the law.