Now, a mediator is not a mediator of one. Some are disposed to philosophize on this expression, and would make Paul's meaning to be, that the twofold nature of Christ is not one in essence. But that Paul is here speaking of the contracting parties, no man of sound judgment entertains a doubt. And so they commonly expound it, that there is no room for a Mediator, unless when one of the parties has a matter to transact with the other. But why that statement should have been introduced they leave undetermined, though the passage manifestly deserves the most careful attention. There may, perhaps, be an Anticipation (prolepsis) of some wicked thought that might arise about a change of the divine purpose. Some one might say, "As men, when they change their mind about their covenants, are wont to retract them, so has it happened with the covenants of God." If you take this to be the meaning, then, in the former clause, Paul would acknowledge that men, who occupy one side of this contract, are unsteady and changeable, while God nevertheless remains the same, is consistent with himself, and partakes not of the unsteadiness of men. But when I take a closer view of the whole subject, I rather think that it marks a difference between Jews and Gentiles. Christ is not the Mediator of one, because, in respect of outward character, there is a diversity of condition among those with whom, through his mediation, God enters into covenant. But Paul asserts that we have no right to judge in this manner of the covenant of God, as if it contradicted itself, or varied according to the diversities of men. The words are now clear. As Christ formerly reconciled God to the Jews in making a covenant, so now he is the Mediator of the Gentiles. The Jews differ widely from the Gentiles; for circumcision and ceremonies have erected "the middle wall of partition between them." (Ephesians 2:14.) They were "nigh" to God, (Ephesians 2:13,) while the Gentiles were "afar off;" but still God is consistent with himself. This becomes evident, when Christ brings those who formerly differed among themselves to one God, and makes them unite in one body. God is one, because he always continues to be like himself, and, with unvarying regularity, holds fixed and unalterable the purpose which he has once made. [1]

Footnotes

1 - "This is confessedly one of the most obscure passages in the New Testament, and, perhaps, above all others, vexatus ab interpretibus,' (tortured by interpreters,) if it be true, as Winer affirms, that there are no less than 250 modes of explanation, most of which are stated and reviewed by Koppe, Berger, Keil, Bonitz, Weigand, and Scheft." -- (Bloomfield.) Schott remarks, that the bare fact of upwards of 250 interpretations makes it impossible to deny that some obscurity attaches to the Apostle's language in this passage, arising chiefly from mere brevity of style, but judiciously adds, that, had there not been many commentators more eager to bring forward anything that has the appearance of novelty, than to investigate the ordinary meaning of the terms, the scope of the passage, and the doctrinal statements and reasonings contained in the writings of the Apostle Paul, the interpretations would never have swelled to so large an amount. -- Ed.

Now a mediator is not a mediator of one - This verse has given great perplexity to commentators. "There is, unquestionably," says Bloomfield, "no passage in the New Testament that has so much, and to so little purpose, exercised the learning and ingenuity of commentators as the present, which seems to defy all attempts to elicit any satisfactory sense, except by methods so violent as to be almost the same thing as writing the passage afresh." In regard, however, to the truth of the declarations here - that "a mediator is not a mediator of one," and that "God is one" - there can be no doubt, and no difficulty. The very idea of a mediator supposes that there are two parties or persons between whom the mediator comes either to reconcile them or to bear some message from the one to the other; and it is abundantly affirmed also in the Old Testament that there is but one God; see Deuteronomy 6:4.
But the difficulty is, to see the pertinency or the bearing of the remark on the argument of the apostle. What does he intend to illustrate by the declaration? and how do the truths which he states, illustrate the point before him? It is not consistent with the design of these notes to detail the numerous opinions which have been entertained of the passage. They may be found in the larger commentaries, and particularly may be seen in Koppe, Excursus vii. on the Galatians. After referring to a number of works on the passage, Rosenmuller adopts the following interpretation, proposed by Noessett, as expressing the true sense. But he (that is, Moses) is not a mediator of one race (to wit, the Abrahamic), but God is the same God of them and of the Gentiles. The sense according to this is, that Moses had not reference in his office as mediator or as internuncius to the descendants of Abraham, or to that one seed or race, referred to in the promise.
He added the hard conditions of the Law; required its stern and severe observances; his institutions pertained to the Jews mainly. They indeed might obtain the favor of God, but by compliance with the severe laws which he had ordained. But to the one seed, the whole posterity of Abraham, they concerning whom the promise was made, the Gentiles as well as the Jews, he had no reference in his institutions: all their favors, therefore, must depend on the fulfillment of the promise made to Abraham. But God is one and the same in reference to all. His promise pertains to all. He is the common God to the Jews and the Gentiles. There is great difficulty in embracing this view of the passage, but it is not necessary for me to state the difficulty or to attempt to show that the view here proposed cannot be defended. Whitby has expressed substantially the same interpretation of this passage. "But this mediator (namely, Moses) was only the mediator of the Jews, and so was only the mediator of one party, to whom belonged the blessing of Abraham, Galatians 3:8, Galatians 3:14. But God, who made the promise, 'That in one should all the families of the earth be blessed,' is one; the God of the other party, the Gentiles as well as the Jews, and so as ready to justify the one as the other."
According to this interpretation, the sense is, that Moses was mediator of one part of Abraham's seed, the Israelites; but was not the mediator of the other part of that seed, the Gentiles; yet there was the same God to both parties, who was equally ready to justify both. Locke has expressed a view of the passage which differs somewhat from this, but which has quite as much plausibility. According to his exposition it means, that God was but one of the parties to the promise. The Jews and the Gentiles made up the other. But at the giving of the Law Moses was a mediator only between God and the Israelites, and, therefore, could not transact anything which would tend to the disannulling of the promise which was between God and the Jews and Gentiles together, the other party to the promise. Or in other words, at the covenant made on Mount Sinai, there was really present but one of the parties, and consequently nothing could be done that would affect the other.
Moses did not appear in behalf of the Gentiles. They had no representative there. He was engaged only for the Jews, for a part only of the one party, and that part could not transact anything for the whole. The giving of the Law, therefore, could not affect the promise which was made to Abraham, and which related to the Jews and the Gentiles as together constituting one party. This view is plausible. It has been adopted by Doddridge, and perhaps may be the true interpretation. No one can deny, however, that it is forced, and that it is far from being obvious. It seems to be making a meaning for the apostle, or furnishing him with an argument, rather than explaining the one which he has chosen to use; and it may be doubted whether Paul would have used an argument that required so much explanation as this before it could be understood. All these expositions proceed on the supposition that the word "mediator" here refers to Moses, and that the transaction here referred to was that on Mount Sinai. I would suggest a sense of the passage which I have found in none of the commentaries which I have consulted, and which I would, therefore, propose with diffidence.
All that I can claim for it is, that it may possibly be the meaning. According to the view which I shall submit, the words here are to be regarded as used in their usual signification; and the simplest interpretation possible is to be given to the propositions in the verse. One proposition is, that a mediator is not appointed with reference to one party, but to two. This proposition is universal. Wherever there is a mediator there are always two parties. The other proposition is, that God is one; that is, that he is the same one God, in whatever form his will may be made known to people, whether by a promise as to Abraham, or by the Law as to Moses. The interpretation which I would propose embraces the following particulars:
(1) The design of the apostle is, to show that the giving of the Law could not abrogate or affect the promise made to Abraham; and to show at the same time what is its true object. It could not annul the promises, says Paul. It was given long after, and could not affect them, Galatians 3:17. It was an addition, an appendage, a subsequent enactment for a specific purpose, yet a part of the same general plan, and subordinate to the Mediator, Galatians 3:19. It was to be shown also that the Law was not against the promises of God. It was a good law Galatians 3:21; and was not designed to be an opposing system, or intended to counteract the promise, or the scheme of salvation by promise, but was a part of the same great plan.
(2) a mediator always supposes two parties. In all the transactions, therefore, where a mediator is employed, there is supposed to be two parties. When, therefore, the promise was made to Abraham with reference to the Messiah, the great Mediator; and when the Law was given in the hand of the Mediator, and under his control, there is always supposed to be two parties.
(3) the whole arrangement here referred to is under the Mediator, and with reference to him. The promise made to Abraham had reference to him and to those who should believe on him; and the Law given by Moses was also under him, and with reference to him. He was the grand object and agent of all. He was the Mediator with reference to both. Each transaction had reference to him, though in different ways the transaction with Abraham relating to him in connection with a promise; the transaction at the giving of the Law being under his control as Mediator, and being a part of the one great plan. There was an identity of plan; and the plan had reference to the Messiah, the great Mediator.
(4) God is one and the same. He is throughout one of the parties; and he does not change. However the arrangements may vary, whether in giving the Law or imparting a promise, He is the same. There is only one God in all the transaction; and He, throughout, constitutes one of the parties. The other party is man, at first receiving the promise from this one God with reference to the Mediator through Abraham, and then receiving the Law through the same Mediator on Mount Sinai. He is still the one party unchanged; and there is the same Mediator; implying all along that there are two parties.
(5) it follows, therefore, agreeably to the argument of the apostle, that the Law given so long after the promise, could not abrogate it, because they pertained to the same plan, were under the same one God, who was one unchanging party in all this transaction, and had reference to the same Mediator and were alike under his control. It followed, also, that the Law was temporary Galatians 3:19; interposed for important purposes until the "seed should come," because it was a part of the same general arrangement, and was under the control of the same Mediator, and directed by the same one God, the unchanging one party in all these transactions. It followed, further, that the one could not be against the other Galatians 3:21, because they were a part of the same plan, under the control of the same Mediator, and where the same God remained unchanged as the one party. All that is assumed in this interpretation is:
(a) That there was but one plan or arrangement; or that the transaction with Abraham and with Moses were parts of one great scheme; and,
(b) That the Mediator here referred to was not Moses, but the Messiah, the Son of God.
The following paraphrase will express the sense which I have endeavored to convey. "The giving of the Law could not annul or abrogate the promise made to Abraham. It was long after that, and it was itself subservient to that. It was given by the instrumentality of angels, and it was entirely under the control of the Mediator, the Messiah. The plan was one; and all the parts of it, in the promise made to Abraham and in the giving of the Law, were subordinate to him. A mediator always supposes two parties, and the reference to the Mediator, alike in the promise to Abraham and in the giving of the Law, supposes that there were two parties. God is one party, the same unchanging God in all the forms of the promise and of the Law. In this state of things, it is impossible that the Law should clash with the promise, or that it should supersede or modify it. It was a part of the one great plan; appointed with reference to the work which the Mediator came to do; and in accordance with the promise made to Abraham; and therefore they could not be contradictory and inconsistent." It is assumed in all this that the Messiah was contemplated in the whole arrangement, and that it was entered into with reference to him. That this may be assumed no one can deny who believes the scriptures. The whole arrangement in the Old Testament, it is supposed, was designed to be ancillary to redemption; and the interpretation which has been submitted above is based on that supposition.

A mediator is not a mediator of one - As a mediator, μεσιτης, signifies a middle person, there must necessarily be two parties, between whom he stands, and acts in reference to both, as he is supposed to have the interests of both equally at heart.
This verse is allowed to be both obscure and difficult; and it is certain that there is little consent among learned men and critics in their opinions concerning it. Rosenmuller thinks that the opinion of Nosselt is to be preferred to all others.
He first translates the words ὁ δε μεσιτης ἑνος ουκ εστιν thus: But he (viz. Moses) is not the mediator of that one race of Abraham, viz. the Christians; for ἑνος relates to the σπερμα ᾡ επηγγελται, the seed that should come, Galatians 3:19, of which he said, ὡς εφ' ἑνος, as of one, Galatians 3:16. If Paul had written ὁ δε μεσιτης του ἑνος εκεινου ουκ εστι, he is not the mediator of one, no person would have had any doubt that σπερματος, seed, ought to be supplied after ἑνος, of one, Galatians 3:19-20. The same mode of speaking Paul uses, Romans 5:17; ὁ δε, but he, ὁ for αυτος, Matthew 12:3, Matthew 12:11, Matthew 12:39, ὁ δε ειπεν, but he said. Though Moses was the Mediator between God and the Israelites, yet he was not the mediator between God and that one seed which was to come; viz. the Gentiles who should believe in Christ.
But God is one - He is the one God, who is the Father of the spirits of all flesh; the God of the Gentiles as well as the God of the Jews. That this is St. Paul's meaning is evident from his use of the same words in other places, 1-Timothy 2:5 : ἑις γαρ Θεος, etc., for there is one God, and one mediator between God and man, that is, there is only one God and one mediator for the whole human race; Ephesians 4:5, Ephesians 4:6 : One Lord, one faith, one baptism, εἱς Θεος και πατηρ παντων, One God and Father of All. The sense of the whole is: Moses was the mediator of one part of Abraham's seed, viz. the Israelites; but of the other seed, the Gentiles, he was certainly not the mediator; for the mediator of that seed, according to the promise of God, and covenant made with Abraham, is Christ.
Though Nosselt has got great credit for this interpretation, it was given in substance long before him by Dr. Whitby, as may be seen in the following words: "But this mediator (Moses) was only the mediator of the Jews, and so was only the mediator of one party, to whom belonged the blessings of Abraham, Galatians 3:8, Galatians 3:14. But God, who made the promise that in one should all the families of the earth be blessed, Is One; the God of the other party, the Gentiles, as well as of the Jews, επειπερ εἱς ὁ Θεος, seeing he is One God, who will justify the circumcision by faith, and the uncircumcision through faith, Romans 3:30." This exposition is so plain, and so well supported by the different scriptures already quoted, that there can be but small, if any, doubt of its propriety. The clause has been translated thus: "Now a mediator supposes two parties, of which God is but one."

Now a mediator is not [a mediator] of one, (24) but God is one.
(24) A taking away of an objection, lest any man might say that sometimes by consent of the parties which have made a covenant, something is added to the covenant, or the former covenants are broken. This, the apostle says, does not come to pass in God, who is always one, and the very same, and like himself.

Now a mediator is not a mediator of one,.... A mediator supposes two parties he stands between, and these at a distance from, or disagreeing with each other; where there is but one party, there can be no need of, nor any reason for, a mediator; so Christ is the Mediator between God and men, the daysman, Job 9:33, that lays his hands upon them both; and Moses, he was the mediator between God and the Israelites:
but God is one; not in person, for there are three persons in the Godhead, the Father, the Word, and the Holy Ghost, and these three are one, in nature and essence; so that though there are three persons, there is but one God, and who is the God both of Jews and Gentiles; who is of one mind concerning them, and has taken them into one and the same covenant, and makes use of one and the same method in the justification of them: but the true sense of the phrase here is, that whereas a mediator supposes two parties at variance, "God is one of the two"; as the Ethiopic version reads the words; he is a party offended, that stands off, and at a distance, which the law given by angels in the hand of a mediator shows; so that that is rather a sign of disagreement and alienation, and consequently that justification is not to be expected by it.

"Now a mediator cannot be of one (but must be of two parties whom he mediates between); but God is one" (not two: owing to His essential unity not admitting of an intervening party between Him and those to be blessed; but as the ONE Sovereign, His own representative, giving the blessing directly by promise to Abraham, and, in its fulfilment, to Christ, "the Seed," without new condition, and without a mediator such as the law had). The conclusion understood is, Therefore a mediator cannot appertain to God; and consequently, the law, with its inseparable appendage of a mediator, cannot be the normal way of dealing of God, the one, and unchangeable God, who dealt with Abraham by direct promise, as a sovereign, not as one forming a compact with another party, with conditions and a mediator attached thereto. God would bring man into immediate communion with Him, and not have man separated from Him by a mediator that keeps back from access, as Moses and the legal priesthood did (Exodus 19:12-13, Exodus 19:17, Exodus 19:21-24; Hebrews 12:19-24). The law that thus interposed a mediator and conditions between man and God, was an exceptional state limited to the Jews, and parenthetically preparatory to the Gospel, God's normal mode of dealing, as He dealt with Abraham, namely, face to face directly; by promise and grace, and not conditions; to all nations united by faith in the one seed (Ephesians 2:14, Ephesians 2:16, Ephesians 2:18), and not to one people to the exclusion and severance from the ONE common Father, of all other nations. It is no objection to this view, that the Gospel, too, has a mediator (1-Timothy 2:5). For Jesus is not a mediator separating the two parties in the covenant of promise or grace, as Moses did, but ONE in both nature and office with both God and man (compare "God in Christ," Galatians 3:17): representing the whole universal manhood (1-Corinthians 15:22, 1-Corinthians 15:45, 1-Corinthians 15:47), and also bearing in Him "all the fulness of the Godhead." Even His mediatorial office is to cease when its purpose of reconciling all things to God shall have been accomplished (1-Corinthians 15:24); and God's ONENESS (Zac 14:9), as "all in all," shall be fully manifested. Compare John 1:17, where the two mediators--Moses, the severing mediator of legal conditions, and Jesus, the uniting mediator of grace--are contrasted. The Jews began their worship by reciting the Schemah, opening thus, "Jehovah our God is ONE Jehovah"; which words their Rabbis (as JARCHIUS) interpret as teaching not only the unity of God, but the future universality of His Kingdom on earth (Zephaniah 3:9). Paul (Romans 3:30) infers the same truth from the ONENESS of God (compare Ephesians 4:4-6). He, as being One, unites all believers, without distinction, to Himself (Galatians 3:8, Galatians 3:16, Galatians 3:28; Ephesians 1:10; Ephesians 2:14; compare Hebrews 2:11) in direct communion. The unity of God involves the unity of the people of God, and also His dealing directly without intervention of a mediator.

Now the mediator is not a mediator of one - There must be two parties, or there can be no mediator between them; but God who made the free promise to Abraham is only one of the parties. The other, Abraham, was not present at the time of Moses. Therefore in the promise Moses had nothing to do. The law, wherein he was concerned, was a transaction of quite another nature.