16. This verse is parenthetical. The covenant of promise was not "spoken" (so Greek for "made") to Abraham alone, but "to Abraham and his seed"; to the latter especially; and this means Christ (and that which is inseparable from Him, the literal Israel, and the spiritual, His body, the Church). Christ not having come when the law was given, the covenant could not have been then fulfilled, but awaited the coming of Him, the Seed, to whom it was spoken.
      promises--plural, because the same promise was often repeated (Ge 12:3, 7; 15:5, 18; 17:7; 22:18), and because it involved many things; earthly blessings to the literal children of Abraham in Canaan, and spiritual and heavenly blessings to his spiritual children; but both promised to Christ, "the Seed" and representative Head of the literal and spiritual Israel alike. In the spiritual seed there is no distinction of Jew or Greek; but to the literal seed, the promises still in part remain to be fulfilled (Ro 11:26). The covenant was not made with "many" seeds (which if there had been, a pretext might exist for supposing there was one seed before the law, another under the law; and that those sprung from one seed, say the Jewish, are admitted on different terms, and with a higher degree of acceptability, than those sprung from the Gentile seed), but with the one seed; therefore, the promise that in Him "all the families of the earth shall be blessed" (Ge 12:3), joins in this one Seed, Christ, Jew and Gentile, as fellow heirs on the same terms of acceptability, namely, by grace through faith (Ro 4:13); not to some by promise, to others by the law, but to all alike, circumcised and uncircumcised, constituting but one seed in Christ (Ro 4:16). The law, on the other hand, contemplates the Jews and Gentiles as distinct seeds. God makes a covenant, but it is one of promise; whereas the law is a covenant of works. Whereas the law brings in a mediator, a third party (Ga 3:19, 20), God makes His covenant of promise with the one seed, Christ (Ge 17:7), and embraces others only as they are identified with, and represented by, Christ.
      one . . . Christ--not in the exclusive sense, the man Christ Jesus, but "Christ" (Jesus is not added, which would limit the meaning), including His people who are part of Himself, the Second Adam, and Head of redeemed humanity. Ga 3:28, 29 prove this, "Ye are all ONE in Christ Jesus" (Jesus is added here as the person is indicated). "And if ye be Christ's, ye are Abraham's SEED, heirs according to the promise."

JFB.