15 Brothers, I speak in human terms: even though it is only a man’s covenant, yet when it has been ratified, no one sets it aside or adds conditions to it.
16 Now the promises were spoken to Abraham and to his seed. He does not say, “And to seeds,” as referring to many, but rather to one, “And to your seed,” that is, Christ.
17 And what I am saying is this: the Law, which came 430 years later, does not invalidate a covenant previously ratified by God, so as to abolish the promise.
18 For if the inheritance is by law, it is no longer by promise, but God has granted it to Abraham through promise.
19 Why the Law then? It was added because of trespasses, having been ordained through angels by the hand of a mediator, until the seed would come to whom the promise had been made.
20 Now a mediator is not for one person only, whereas God is one.
21 Is the Law then contrary to the promises of God? May it never be! For if a law had been given which was able to impart life, then righteousness would indeed be by law.
22 But the Scripture has shut up everyone under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
“The one and only heir of every promise of God is Christ. Every promise given in the covenant with Abraham was fulfilled in Jesus Christ and only Jesus Christ. Therefore the only way a person can participate in the promised blessings to Abraham is to be a fellow heir with Christ through faith in Him.” (Dr. John MacArthur)
“Such was God’s promise. It was free and unconditional. As we might say, there were ‘no strings attached’. There were no works to do, no laws to obey, no merit to establish, no conditions to fulfill. His promise was like a will, freely giving the inheritance to a future generation. And like a human will, this divine promise is unalterable. It is still in force today, for it has never been rescinded. God does not make promises in order to break them. He has never annulled or modified His will." (John R. W. Stott)
“Satan would have us to prove ourselves holy by the law, which God gave to prove us sinners.” (Andrew Jukes)