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.
THE COVENANT AND THE LAW
We must distinguish between God’s Covenant and the Law:
Time difference – God made the Covenant with Abraham and with his “seed.” The Law, with its myriads of rules and regulations, was added 430 years later through Moses.
Before Moses – God’s Covenant with Abraham, which included conditions and blessings, was not conditional on compliance with the Law of Moses. Abraham’s children (Isaac, Jacob, etc.), for example, did not have to comply with the Law of Moses to inherit the covenant blessings for the law did not even yet exist.
Law Added – When the Law was added to the covenant through Moses, it did not nullify or replace the covenant. The covenant remained the fundamental binding agreement; similar to the constitution of a nation. The covenant blessings which God promised to Abraham did not become conditional on the “works of the Law.” It was rather the other way round; the Law of Moses must always be interpreted on the basis of the covenant.
All nations – God said to Abraham, “All the nations will be blessed in you” (Gal 3.8). In other words, God’s covenant is with all of His people; including Gentiles. The Law of Moses, on the other hand, belongs specifically to the nation of Israel.
In Christ – “The promises were spoken to Abraham and to his seed” (3:16). The promises refer to the Covenant which God made with Abraham (Genesis 12, 22:17). Jesus Christ is Abraham’s true “seed.” As such, when He came, He inherited the covenant blessings. However, He did not inherit the Law of Moses.
Jewish Christians came to Galatia and argued that, to be redeemed, one must comply with the Law of Moses. Gentile Christians, they said, must, therefore, also be circumcised and comply with the Law of Moses.
In the current verses, Paul responds and argues that the Covenant blessings were based on God’s promise and are not conditional on compliance with the Law of Moses.
Furthermore, since Jesus is Abraham’s true “seed,” He inherited the Covenant promises (3:16-19). He is the “heir of all things” (Hebr. 1:2). Thus everything belongs to Christ. And the only way in which 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). Then they share in the covenant promises. In other words, the inheritance is not received through the Law.
All of this is still part of Paul’s explanation that man is justified through faith in Christ Jesus (2:16); not on the basis of the Law of Moses.