7-9. Neither, because they are the seed of Abraham, are they all children--"Not in the line of mere fleshly descent from Abraham does the election run; else Ishmael, Hagar's child, and even Keturah's children, would be included, which they were not."
      but--the true election are such of Abraham's seed as God unconditionally chooses, as exemplified in that promise.
      in Isaac shall thy seed be called-- (Ge 21:12).

JFB.