This is a text acknowledged by all interpreters to be very obscure; not so much as considered in itself, (for all know, that a mediator speaks one that goes in the middle between two persons that are at odds, so cannot be of one), as in regard of the connection of it with what went before; where he had told us, that the law was given in the hand of a mediator. There are various senses given of this verse, and the variety much ariseth from men’s different understanding of the mediator in whose hand the law was given. To me the apostle seems to magnify the promise above the law, in that the promise was given to Abraham immediately by God, (who is one in essence), but the law was given not immediately by God, but by Moses as mediator, who in that action was a type of Christ. And God thereby showed, that the law would bring no man to life and salvation without the one and only Mediator Christ Jesus. Christ, indeed, is the Mediator of the new testament, he mediated for it, he mediateth in it; but it was men’s transgression of the law that brought them in need of a Mediator, sin being the only thing that separateth between God and man.
God is one; and there had been no need of mediating between him and man, but for the law which man had transgressed. Those that by the mediator, Gal_3:19, understand Christ, make this the sense: That as a mediator supposeth two parties at odds, so Christ’s being Mediator speaks him to have respect to Jews and Gentiles. But this interpretation seems to make Christ the Mediator between Jews and Gentiles, whom (the apostle saith) he made both one, breaking down the partition-wall, Eph_2:14; but we do not find the name of Mediator upon this account any where given unto Christ. Many other senses are given, but the first mentioned seemeth the most probable, viz. that God made use of no mediator in giving the promise, but only in giving the law, which evidenced that justification was not to be by it; nor had there been need of a true Mediator under the gospel, but for the law, men’s transgression of which brought in a need of a Mediator; which proved that justification could not be by the law.