Hebrews 11:18

Verse 18. Of whom it was said, That in Isaac shall thy seed be called. Ge 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 bond-woman 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, he 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.