Of whom it was said, That in Isaac shall thy seed be called:
Of whom - `He (Abraham, not Isaac) TO whom it was said' (Alford). But Hebrews 1:7 uses the same х pros (G4314)] 'unto' for 'in respect to,' or "of," as the English version. This verse defines "only-begotten son," Hebrews 11:17.
In Isaac shall thy seed be called (Genesis 21:12). The posterity of Isaac alone shall be accounted the seed of Abraham, which is heir of the promises (Romans 9:7).