A.T. Robertson Commentary Galatians 3:16

"Now to Abraham were the promises spoken, and to his seed. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ." — Galatians 3:16 (ASV)

But as of one (αλλ' ως εφ' ενος). But as in the case of one.

Which is Christ (ος εστιν Χριστος). Masculine relative agreeing with Χριστος though σπερμα is neuter. But the promise to Abraham uses σπερμα as a collective substantive and applies to all believers (both Jews and Gentiles) as Paul has shown in verses 7-14, and as of course he knew full well Here Paul uses a rabbinical refinement which is yet intelligible. The people of Israel were a type of the Messiah and he gathers up the promise in its special application to Christ. He does not say that Christ is specifically referred to in Ge 13:15 or 17:7f.