Note 22 at Gal. 3:16: Paul has been arguing from verse 15 that the promise or covenant given to Abraham is still in force because no man can add to or take away from a legal covenant.

The promise or covenant was made with Abraham and his "seed" (singular-Gen.12:2-3; 13:14-15; 15:1, 5-6, 18; 17:2, 7; Rom. 4:6-9; 13-14; 22-25; Gal. 3:14, 22, 29). The promises were not given to Abraham and his "seeds" (plural-referring to the Jewish people, his descendants) but to his "seed"; his ONE descendant, which is Christ.

The Greek word for "seed" is SPERMA, which means "the seed, i.e. the grain or kernel which contains within itself the germ of the future plants" (Thayer). Paul is going to later argue that believers were the future plants, so to speak, that were "in" the one seed-Christ. "Thus, to Abraham personally and to all those who by faith in Christ are brought into salvation, were the promises made" (Wuest's Word Studies, Vol. 1, p.101; Gal. 3:29). God's promise of justification through faith was given long before the introduction of the law and always has been the only way to justification with God.