Now a mediator is not a mediator of one, but God is one.
'Now a mediator [in the essential idea: ho (G3588) mesitees (G3316): the article is generic] cannot be of one (but must be of two parties whom he mediates between); but God is one' (not two: His unity admits not an intervening party between Him and those to be blessed: as the ONE Sovereign, His own representative, He gives the blessing directly by promise to Abraham, and, in its fulfillment, to Christ, 'the Seed,' without new conditions, and without a mediator such as the law had). He recognizes no second party (as man) dealing on independent terms with Him through a mediator. The conclusion understood is, Therefore a mediator cannot appertain to God; consequently, the law, with its inseparable appendage of a mediator, and two parties to be mediated between in the way of compact, cannot be the normal way of dealing of God, who acts singly and directly. God would bring man into immediate communion, and not have man separated from Him by a mediator, as Israel was by Moses and the legal priesthood (Exodus 19:12-13; Exodus 19:17; Exodus 19:21-24; Hebrews 12:19-24).
The law that interposed a mediator and conditions between man and God was an exceptional state limited to the Jews, parenthetically preparatory to the Gospel, God's normal dealing, as He dealt with Abraham-namely, face to face directly, by promise and grace, not conditions; to all nations united by faith in the one seed (Ephesians 2:14; Ephesians 2:16; Ephesians 2:18); not to one people, to the exclusion of all others from the ONE common Father. It is no objection that the Gospel, too, has a mediator (1 Timothy 2:5); for Jesus is not a mediator separating the two parties as Moses did, but ONE in nature and office with both God and man (cf. "God in Christ," Galatians 3:17), representing the universal manhood (1 Corinthians 15:22; 1 Corinthians 15:45; 1 Corinthians 15:47), and bearing "all the fullness 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 (Zechariah 14:9) as 'all in all' shall be fully manifested.
Compare John 1:17, where Moses, the severing mediator of legal conditions, and Jesus, the uniting mediator of grace, are contrasted. The promise is called 'a covenant' (Galatians 3:17), because settled in the eternal counsels of the triune God, and depending on conditions fulfilled by the Son of God. This covenant was carried out in the unity of God, without a mediator. It supposes only one party (not two, as the law): God is that One. Himself, in Christ, being the mediator. Paul's argument pre-supposes Christ's Godhead; otherwise He would be a mediator distinct from God, as was Moses, and the argument would fail. The Jews began worship by reciting the Schemah, opening thus-`Yahweh our God is ONE Yahweh;' 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 (cf. Ephesians 4:4-6). He, being one, unites all believers, without distinction, to Himself (Galatians 3:8; Galatians 3:16; Galatians 3:28; Ephesians 1:10: cf. Hebrews 2:11) in direct communion. The unity of God involves the unity of God's people, and also His dealing directly without intervention of a mediator.