What does Galatians 3:20 mean?

Now a mediator is not a mediator of one, but God is one.

 

In context, Paul is arguing the inferiority of the law, and the superiority of grace. Justification is by grace alone, not by grace plus works, because the law is inferior, it only condemns, it does not provide enough to live off. (10-12)

For all who rely on works of the law are under a curse; for it is written, “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them.”Now it is evident that no one is justified before God by the law, for “The righteous shall live by faith.” But the law is not of faith, rather “The one who does them shall live by them.”

Paul then argues the promise aspect of the Covenant was superior to the law because the promise includes the Gentiles. (13-14).

Christ redeemed us from the curse of the law by becoming a curse for us—for it is written, “Cursed is everyone who is hanged on a tree”— so that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we might receive the promised Spirit through faith.

Now a clever Judiazer might come along and argue that the promise was superseded by the law, since the law came later. Paul cuts this off. (15-19)

To give a human example, brothers even with a man-made covenant, no one annuls it or adds to it once it has been ratified. Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise. Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by a mediator.

So given the context of what has come before, in verse 20 Paul must be saying something to the effect of the law is inferior to the promise. Unfortunately that doesn't narrow it down much.

Now a mediator is not a mediator of one, but God is one.

 

Possible Meaning One

Hypothetical antagonistic Judiazer is back to argue that the law did indeed amend the promise to Abraham. "Ah yes," he would say "the mediator is present during the giving of the law, which means both parties are present, therefore the agreement is open for modification. The law is indeed superior to the promise."
Verse 20 is Paul cutting that argument down by affirming their premise "Yes a mediator is not a mediator of one, both parties are present" but denying their conclusion "but remember God is one with His son so He wouldn't negotiate against the will of the Father. The law remains inferior to the promise."

What I like about this argument: Paul has been anticipating the judiazer arguing, v19 for example, "What was the purpose of the law" v17 "And this I say," and it would make sense he keeps this going.

Now a mediator is not a mediator of one, but God is one.

Possible Meaning Two

Moses is the mediator spoken of here as in verse 19, and this is yet another argument stacked up to prove the inferiority of the law. In this case the law is weak because it requires another party on which to depend, a mediator, because a mediator is not a mediator of just one, while the promise does not need anyone else, because God is one. Therefore, because the promise stands alone and the law doesn't, the law is inferior.

What I like about this argument: It ties back strongly into v18 "God gave it to Abraham by a promise."   

Now a mediator is not a mediator of one, but God is one.

Possible Meaning Three

Lock's view is that the Gentiles are chiefly what Paul has in mind. On Sinai Moses mediated the relationship between God and Israel, but the Gentiles, who were included in the promise to Abraham, are left out.
Therefore "a mediator is not a mediator of one" is a denial that Moses is a mediator with legal standing to re-write the agreement, because he only stands for one party, Israel, and not both parties. Moses therefore cannot really be a mediator, even though God is there, so he’s the one that counts for His side. Therefore, the law cannot supersede the promise, because Moses has no standing, even though God is present.

What I like about this argument: It lines up very nicely with the larger context of justification by faith, and the Gentiles being Abrahams sons in v14. It also lines up very well with Romans 3:29-30 "Is he the God of the Jews only? Is he not also of the Gentiles? Yes, of the Gentiles also: Seeing one God, which shall justify the circumcision by faith, and uncircumcision through faith.”

Now a mediator is not a mediator of one, but God is one.

Possible Meaning Four

Hypothetical Judiazer is at it again, "If Christ is the mediator for humanity against God, then how can He be God?" In saying verse twenty Paul merely affirms the Trinitarian view that Jesus is both a mediator, and yet at the same time God.

What I like about this argument: It's simple.

Anything I missed?