Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Note 22 at Ga 3:16: Paul had been arguing from Ga 3:15 that the promise, or covenant, given to Abraham was still in force because no one can add to or take away from a legal covenant.
The promise, or covenant, was made with Abraham and his "seed" (singular - Ge 12:2-3, 13:14-15, 15:1, 5-6, 18, 17:2, 7; Ro 4:6-9, 13-14, 22-25; Ga 3:14, 22, and 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, who is Christ.
The Greek word for "seed" is "SPERMA," and it means "'the seed' i.e. the grain or kernel which contains within itself the germ of the future plant" (Thayer's Greek-English Lexicon). Paul would 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" ("Galatians in the Greek New Testament" by Kenneth S. Wuest, p. 101; see also Ga 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.