One God and one mediator
Here Jesus is distinguished from the one God, meaning that Jesus is someone other than the one and only God. Paul does this in two ways.
Firstly, Paul refers to two distinct parties, the one God and the one mediator. The fact Paul makes two statements—God is one, and the mediator is one—implies that these really are two distinct parties, not merely two names for the same party (which could be expressed as “there is one God and mediator”).
But what about two distinct parties that overlap? For example, Jesus and the Trinity. Well, Paul's second way of distinguishing Jesus and the one God rules this out too. He calls Jesus the mediator between the one God and (the rest of) mankind. By definition, a mediator or intermediary cannot be a member of one of the parties they are mediating for; a mediator is a middle man who bridges the gap between two or more parties, allowing the parties to interact indirectly. If a mediator were a member of one of the parties they were mediating for then the parties would be interacting directly without mediation.
Gal 3:19-20 teaches much the same, and is relevant to how God relates to His mediators. The same word μεσίτης (mesites) is used in Gal 3:19, 20, and 1 Tim 2:5. These verses tell us that although God can work through a mediator, “God is one” and “the mediator is not of one”, meaning either that God and the mediator together make more than one, or that the mediator is not part of any one party, let alone part of God, because God is one. Thayer's lexicon corroborates this interpretation: “every mediator, whoever acts as mediator, ἑνός οὐκ ἐστι, does not belong to one party but to two or more, Galatians 3:20. Used of Moses, as one who brought the commands of God to the people of Israel and acted as mediator with God on behalf of the people, Galatians 3:19”.
So then, the one mediator between the one God and mankind cannot be a member of the one God. Although Jesus is a member of mankind, it is implied that the verse is only referring to the rest of mankind; Jesus can hardly be a mediator between God and Himself, nor does He need such a mediator. Consequently, the best way to interpret the “one God” is to understand Him to be the Father, in agreement with the rest of the epistle (1 Tim 1:17, 6:15-16) and other scriptural uses of the phrase εἷς θεός (“one God”; 1 Cor 8:6, Eph 4:6, Mark 12:32, Mal 2:10 Gk.).
- Christ’s mediation is the fulfillment and end of all mediation. The book of Hebrews opens with the assertion that Christ surpasses all other mediators—angels, Moses, and the priests of the Aaronic priesthood. Christ’s priesthood is a timeless priesthood, like Melchizedek’s. The new covenant He established between God and believers offers better promises, a better sacrifice, a better sanctuary, and a better hope (Heb. 7:19; 8:6; 9:1, 11–15). Christ’s mediation excels all others and can never be superseded. He is the only way to God; there is no other way—no other “go-between.” Carpenter, E. E., & Comfort, P. W. (2000). In Holman treasury of key Bible words: 200 Greek and 200 Hebrew words defined and explained (p. 333). Nashville, TN: Broadman & Holman Publishers. It is a mistake to place human limitations on God. God can and did act as a mediator between humanity and himself (John 3:16).