What is the meaning of Gal. 3: 20: “But a mediator is not of one, but God is one”?
Before looking at the verse in question, we must take careful account of the context and also the thrust of the whole epistle. The latter is expressed very early in Gal. 1: 6–9. There were those in Galatia who were adding to the Gospel as shown by the Apostle’s use of the word besides in vs 8, 9. Adding to the Gospel effectively annulled its truth. They were not trying to replace grace with law, but seeking to add law to grace. The Apostle uses the strongest language in any of his letters to condemn this. It also appears that there were those who were seeking to make Gentile brethren into Jews, forcing them to Judaize (Gal. 2: 14). This involved separation when eating, observation of days, and circumcision (see Gal. 2: 12; 4: 10; 5: 2).
In chapter 3 Paul turns to Abraham and the scriptural testimony recorded in Gen. 15: 6 that “Abraham believed God, and it was reckoned to him as righteousness” (Gal. 3: 6). This was long before the the law was given at Sinai and is the essence of the Gospel (see v8), namely justification by faith. That this blessing would be universal, and not restricted to those under law, is shown by the further word to Abraham when God promised “In thee all the nations shall be blessed” (v8). But then the question arises, Why the law? This brings us to the section in question (vs 15–20).
Now Gal. 3: 15–20 contains words that we must be very clear as to their meaning. A covenant is an agreement between two parties. The Bible contains a number of covenants. Some are between man and man; others between God and man. Those between God and man may be conditional or unconditional. A covenant may require a mediator, that is one who can stand between the two parties and yet have dealings with both. If a covenant requires a mediator, then there is disparity or disagreement between the parties. Such was the covenant of law given at Sinai, having Moses as its mediator. (Again, the new covenant, yet to be made with Israel and Judah, has Christ as Mediator—see Heb. 9: 15—but the new covenant is never the subject in Galatians). With the Abrahamic covenants there is absolutely no thought of a mediator and when these are taken up by Paul in the NT they are described as “the covenants of promise” (Eph. 2: 12). Indeed, he always uses the word promise in connection with them (see Rom. 4: 13, 14, 16, 20, 21; 9: 8, 9; Heb. 6: 15, 17). Why? Because a promise depends for its fulfilment only on the one who made it and this emphasises the fact that the Abrahamic covenants were unconditional and depended for their fulfilment on only one of the two parties, that is, on God alone.
Turning now to Gal. 3: 15–20, the Jewish legalisers would acknowledge that the covenants made with Abraham were authored by God. They would also contend that the law given at Sinai was given by God and thus given in addition. But Paul insists that the Abrahamic covenants are entirely independent of the law. For even among men, no one would think of adding other dispositions to a confirmed covenant (see v 15), and the Abrahamic covenants were confirmed by both word and oath (see Heb. 6: 17, 18). Now while Abraham’s seed, spoken of in these covenants, is identified with a multitude using the figures of the dust of the earth, the sand of the sea shore and the stars of heaven to express its magnitude (see Gen 13: 16; 22: 17 etc.), it is also identified with singularity—“thy seed” (Gen. 22: 18). It is this oneness that the Apostle takes up here—not “as of many; but as of one” (Gal. 3: 16), that is Christ. Now it is this singularity that is the key to the interpretation of v20. In the Abrahamic covenants, universal blessing for the nations is always identified with the promise to Abraham and the promise to the seed (see Gen. 12: 3; 18: 18; 22: 18; 26: 4). This has nothing to do with the law. “Why then the law? It was added for the sake of transgressions …” (Gal. 3: 19), so that sin might be put to account (see Rom. 5: 13), and that its true character may be made manifest (see Rom. 7: 13). It was added “until the seed came” (Gal. 3: 19)—it was temporary until Christ. Clearly the law also had Moses as a mediator, and since there was a mediator, there were two parties and furthermore disparity between them. But the promises to Abraham had no mediator, and rested for their fulfillment on one party, that is God—“God is one” (v20, my emphasis). Again, if the law annulled the promises so as “to make the promise of no effect” (v17) then there would be discord between promise and law. But God was the author of both and He cannot disagree with Himself for “God is one” (v20, my emphasis).