Now a mediator is not a mediator of one (ὁ δὲ μεσίτης ἐνὸς οὐκ ἔστιν)
Observe, 1. Δὲ is explanatory, not antithetic. The verse illustrates the conception of mediator. 2. The article, the mediator, has a generic force: the mediator according to the general and proper conception of his function. Comp. the apostle (2Co 12:12); the shepherd, the good (Joh 10:11). 3. Ἑνὸς of one, is to be explained by the following εἷς, so that it is masculine and personal. We are not to supply party or law. The meaning is: the conception of mediator does not belong to an individual considered singly. One is not a mediator of his single self, but he is a mediator between two contracting parties; in this case between God and the people of Israel, as Lev 26:46; thus differing from Christ, who is called the mediator of a new covenant (Heb 8:6; Heb 9:15; Heb 12:24). The new covenant, the gospel, was not a contract. Accordingly Gal 3:20 serves to define the true conception of a mediator, and through this definition to make clearer the difference between the law, which required a mediator, and the promise, which is the simple expression of God's will. The very idea of mediation supposes two parties. The law is of the nature of a contract between God and the Jewish people. The validity of the contract depends on its fulfillment by both parties. Hence it is contingent, not absolute.
But God is one (ὁ δὲ θεὸς εἷς ἀστίν)
God does not need a mediator to make his promise valid. His promise is not of the nature of a contract between two parties. His promise depends on his own individual decree. He dealt with Abraham singly and directly, without a mediator. The dignity of the law is thus inferior to that of the promise.