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