Galatians 3:17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
18 For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.
This is saying, that even though the law was given to Moses about 430 years after this promise was made to Abraham, it does not fulfill the promise made to Abraham, or do away with it. God had to bring them this way, so they could see that law alone would not save anyone. In the 430 years between the giving of this covenant and the law’s appearance, God justified man by faith. If the keeping of the law could bring the inheritance, it would not be an inheritance. An inheritance is something you receive at the death of another, which you have not earned. It is given to you because of your relation to the one who died. This shows, not only the greatness of the inheritance, but the greatness of the giver of the inheritance. God, through Jesus Christ, willed us the great inheritance. It is ours by sonship. All of God’s promises are ours and we need to claiming them.