In Galatians 3, Paul was shocked that Christians would allow themselves to let a leader impose rules when they had been set free by the Gospel of Jesus Christ.
At the end of the chapter, Paul gave an example. He said that it was a human example, easy for them to understand. He said that even with a man-made agreement, nobody can annul it or add to it once it has been agreed upon.
The promises given to Abraham were made to Abraham and to his offspring. It does not say to Abraham's offsprings (plural)! It was not referring to many, but to One. That One, Paul said, was Jesus Christ.
The Law, which came 430 years after this covenant, does not annul the previous covenant to make it void. If the inheritance comes by Law, than it no longer comes by promise. But God gave it to Abraham by a promise!
Why then do we have the Law? Paul said it was to put transgressions in place until the Offspring would come, the One for which the Promise had been made to Abraham. Angels were put in place by an intermediary, but now that Christ has came, He is the One intermediary.
Paul asked, "Is the Law then contrary to the promises of God?" He said "Certainly not!" The scriptures imprisoned everything under sin, so that the promise by faith in Jesus Christ could be given to those who believe.
The Law was necessary. We were held captive under the Law, imprisoned until the coming Faith would be revealed. The Law was the guardian until Christ came, so that we could be justified by faith.
Paul said now that faith has come, we are no longer under that guardian! We are no longer under the Law! In Christ Jesus, we are all sons of God through faith.
There is neither Jew nor Greek, neither slave nor free, neither male nor female in God's eyes. We are all one in Christ Jesus. And if we are in Christ, we are also in Abraham's offspring, heirs to the promise!
As Christians, we should ask ourselves: Have we let others bind us with rules instead of the Gospel of Jesus Christ? Have we rejected the promise to Abraham?