Promise & Law
When coming to God, it is important to understand our access. It is through covenant, which has been God's use throughout much of His dealings with humankind.
Paul when comparing the two covenants, briefly explains how they operate:
15Brothers, speaking of human terms, though it is only a man’s covenant, yet when it has been confirmed, no one makes it void, or adds to it. Galatians 3:15
Referring to the covenants of man, and how they operated, Paul shows how even our understanding can reveal how God is working with His people.
No one is able to cancel a contract by himself, nor can he add to it, once it has been confirmed. To that which the two parties have agreed cannot be changed.
When Yahweh cuts a covenant with either a person or a people, it stands fast until the terms are met. Nothing can be added to it while it is in effect.
The apostle Paul here uses this description to show that the original promises made to Abraham stand fast. God promised certain things to his Seed, with no further requirement—Abraham simply believed, and he was accounted righteous for it.
The Law of Moses came some 400 years later. It could not be added to that original covenant between Yahweh and Abraham, the promise to his seed, because a covenant cannot be amended. Nor could that first covenant with Abraham be annulled when the newer covenant of Moses with Israel came to pass.
17Now I say this: A covenant confirmed beforehand by God in Christ, the law, which came four hundred thirty years after, does not annul, so as to make the promise of no effect. Galatians 3:17
There were two covenants, which Paul describes with two words, "promise" and "law." The first, with Abraham, was of promise—Abraham simply believed Yahweh as to what was promised and he received. The second, with the children of Israel through Moses, was of law—that is, they were required to live a certain way, do certain works, to fulfill the covenant.
The two cannot be mixed, nor does one annul the other. So we see how covenants operate, they are self-sufficient and stand on their own.
18For if the inheritance is of the law, it is no more of promise; but God has granted it to Abraham by promise. Galatians 3:18
Paul reveals that the original covenant, with Abraham, applies to all those who are of the Seed to which the promise came. It was to Abraham's Seed—singular, not plural, seeds—revealing it spoke of Christ.
16Now the promises were spoken to Abraham and to his offspring. He doesn’t say, “To descendants”, as of many, but as of one, “To your offspring”, which is Christ. Galatians 3:16
As we have come to understand through the enlightment of Yahweh's Spirit, all those who believe and receive God's Son, Christ Jesus, become one with Him. Therefore, being one with that Seed, Christ, they receive what was promised to that Seed. The later covenant which lasted for some time with Israel through Moses, cannot and does not apply to those who are under the covenant with Abraham's Seed.
26For you are all children of God, through faith in Christ Jesus. 27For as many of you as were baptized into Christ have put on Christ. 28There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus. 29If you are Christ’s, then you are Abraham’s offspring and heirs according to promise. Galatians 3:26-29