Brothers and sisters, let me use an example from everyday life. No one can cancel a person's will or add conditions to it once that will is put into effect.
The promises were spoken to Abraham and to his descendant. Scripture doesn't say, "descendants," referring to many, but "your descendant," referring to one. That descendant is Christ.
This is what I mean: The laws [given to Moses] 430 years after God had already put his promise [to Abraham] into effect didn't cancel the promise [to Abraham].
If we have to gain the inheritance by following those laws, then it no longer comes to us because of the promise. However, God freely gave the inheritance to Abraham through a promise.
What, then, is the purpose of the laws given to Moses? They were added to identify what wrongdoing is. Moses' laws did this until the descendant to whom the promise was given came. It was put into effect through angels, using a mediator.
A mediator is not used when there is only one person involved, and God has acted on his own.
Does this mean, then, that the laws given to Moses contradict God's promises? That's unthinkable! If those laws could give us life, then certainly we would receive God's approval because we obeyed them.
But Scripture states that the whole world is controlled by the power of sin. Therefore, a promise based on faith in Jesus Christ could be given to those who believe.
We were kept under control by Moses' laws until this faith came. We were under their control until this faith which was about to come would be revealed.
Before Christ came, Moses' laws served as our guardian. Christ came so that we could receive God's approval by faith.
But now that this faith has come, we are no longer under the control of a guardian.