{But as of one} (all' h(9373) eph' henos). But as in the case
of one. {Which is Christ} (hos estin Christos). Masculine
relative agreeing with Christos though sperma is neuter. But
the promise to Abraham uses sperma 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 .