3:15 Brethren, I speak in terms of human relations: 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. 3:16 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. 3:17 What I am saying is this: the Law, which came four hundred and thirty years later, does not invalidate a covenant previously ratified by God, so as to nullify the promise. 3:18 For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise.
Paul now provides two reasons for his statement above that man is justified through faith in Christ Jesus (2:16):
Firstly, above he interpreted the promise to Abraham, that “all the nations will be blessed in you“, as that “God would justify the Gentiles by faith” (3:8). Now he adds that this promise was received hundreds of years before the law, and therefore the law did not invalidate the promise (3:8, 15, 17-18).
Secondly, the promises, which God made Abraham, were actually received by Christ (3:16, 19), who He appointed as “heir of all things” (Hebr. 1:2). Thus everything belongs to Christ. The only way that people can become “Abraham’s descendants, heirs according to promise” (3:29) is “in Christ Jesus” (3:14), which means that they “belong to Christ” (3:29).
Inheritance (3:18): – 3:8 interprets the promises to Abraham as a promise that Gentiles will be justified by faith. 3:18 again refers to the promises, but says that an “inheritance” was promised to Abraham (compare 3:29). One therefore receives the inheritance when one is justified by faith. Or, stated differently. It is the people that are justified by faith that will receive the “inheritance”.