Hebrews 11:18

“Of whom it is said, etc..] That is, of Isaac, Abraham's own son, whom he offered up; or rather, “to whom it was said”, as the Vulgate Latin and Syriac versions render it; that is, to Abraham, for to him was this said, (<012112>Genesis 21:12) that in Isaac shall thy seed be called; that numerous natural seed of his, which should inherit the land of Canaan; and his special famous seed, the Messiah, to whom the promises were made; (see Gill on “ <450907>Romans 9:7”).”

“Of whom--rather as Greek "He (Abraham, not Isaac) TO whom it was said" [ALFORD]. BENGEL supports English Version. So Heb 1:7 uses the same Greek preposition, "unto," for "in respect to," or "of." This verse gives a definition of the "only-begotten Son" (Heb 11:17). in Isaac shall thy seed be called-- (Gen 21:12). The posterity of Isaac alone shall be accounted as the seed of Abraham, which is the heir of the promises (Rom 9:7).”

“Of whom it was said, That in Isaac shall thy seed be called; - Genesis 21:12. A numerous posterity had been promised to him. It was there said expressly that this promise was not to be fulfilled through the son of Abraham, by the bondwoman Hagar, but through Isaac. Of course, it was implied that Isaac was to reach manhood, and yet notwithstanding this, and notwithstanding Abraham fully believed it, be prepared deliberately, in obedience to the divine command, to put him to death. The phrase “thy seed be called” means, that his posterity was to be named after Isaac, or was to descend only from him. The word “called” in the Scriptures is often equivalent to the verb “to be;” see Isaiah 56:7.…”

“In Isaac shall thy seed be called - From him shall the blessed seed spring. Gen 21:12. Hebrews 11:19”

“Of: or, To That: Gen 17:19, Gen 21:12; Rom 9:7 Hebrews 11:19”