Dear brothers and sisters, here's an example from everyday life. Just as no one can set aside or amend an irrevocable agreement, so it is in this case.
References for Galatians 3:15
God gave the promise to Abraham and his child. And notice that it doesn't say the promise was to his children, as if it meant many descendants. But the promise was to his child -- and that, of course, means Christ.
References for Galatians 3:16
This is what I am trying to say: The agreement God made with Abraham could not be canceled 430 years later when God gave the law to Moses. God would be breaking his promise.
For if the inheritance could be received only by keeping the law, then it would not be the result of accepting God's promise. But God gave it to Abraham as a promise.