This verse, closing the short paragraph commencing the verse which precedes it, appears designed to mark the difference of the relations which subsisted between the Lord and Israel at the time of the giving of the Law, compared with those which subsist between God and Abraham's seed in the covenant of grace.
Now a mediator is not a mediator of one ( ὁ δὲ μεσίτης ἑνὸς οὐκ ἔστιν). The article with μεσίτης, literally, "the mediator," marks the noun as a class noun, giving it the sense, "a mediator as such."
Compare the use of the article in τοῦ ἀποστόλου, in "the signs of an apostle" (2 Corinthians 12:12); in ὁ ἀγαθὸς ἄνθρωπος, "a good man" (Matthew 12:25); in ὁ ἐργάτης, "the labourer is worthy of his hire" (Luke 10:7).
The clause means this: a mediator implies the existence of more than one party, of two parties at least, for him to mediate between; of two parties not at one, but standing on such terms towards each other as make his intervention necessary.
So far as it characterized the giving of the Law viewed in contrast with the establishment of the covenant of grace, the mediation of Moses, as has been already observed, did not put an end to the estrangement between the Lord and Israel: the estrangement went on throughout Moses' life; throughout, the Israelites stand marked with the brand of "transgression."
The genitive ἑνός, "of one," is the same as the genitive in μεσίτης θεοῦ καὶ ἀνθρώπων, literally, "Mediator of God and men," in 1 Timothy 2:5 : it marks the party or parties towards whom the function of mediation is exercised; so that what the apostle here affirms is that there cannot be only one such party.
But God is one ( ὁ δὲ θεὸς εἷς ἔστιν). When we consider the number of interpretations given of this clause in connection with the preceding, which have literally been computed by hundreds (the reader will find a spieilegium of some sixty or eighty of them in Meyer), we may infer with certainty that the sense which the apostle intended to convey is not an obvious one—not one which lies near the surface.
So much appears, however, in the highest degree probable, that he refers either to some disadvantageous circumstance attaching to the Law or to some advantageous circumstance attaching to the covenant of promise, and is viewing the two in contrast the one with the other.
On these grounds the present writer has long since acquiesced in the view propounded by Windischmann in his Commentary on this Epistle, and which is accepted by Bishop Ellicott, that the unity here predicated of God is the oneness subsisting between the Father and the Son.
God is one in the Father and in his Son—Christ our Lord. The fact is now present to the apostle's mind, and is presently after stated by him (Galatians 4:4), that the Son has been "sent forth" by God to redeem us and make us sons, and has thus become the "Christ," that "Seed of Abraham" to which the promises had been made.
Hereby the most perfect oneness is established between God and the heirs of the promise; for these are "clothed with Christ" (verse 27) the Son of God; and he being one with the Father, they in and through him are really and permanently "reconciled into God," as the apostle writes in Colossians 1:20.
Compare our Lord's words in his intercessory prayer (John 17:21, John 17:23), "That they all may be one; even as thou, Father, art in me, and I in thee, that they also may be in us. I in them, and thou in me; that they may be perfected into one."
That this sense lies deep down in the apostle's words and would not have readily been presented by them to the minds of his readers, forms no valid objection to this interpretation; for the history of the exegesis of the passage proves that this must have been the case with the sense which the apostle really designed to indicate, whatever that was.
On the other hand, it is a sense which perfectly suits the requirement of the context; for it illustrates the superiority of the covenant of the promise to the covenant of the Law in the strongest manner possible.
The nut has a very hard shell, but it yields a delicious kernel.