Albert Barnes Commentary Hebrews 11:18

"even he to whom it was said, In Isaac shall thy seed be called:" — Hebrews 11:18 (ASV)

Of whom it was said, That in Isaac shall your 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 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, "your 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. To name or call a thing was the same as to say that it was, or that it existed. It does not mean here that his spiritual children were to be called or selected from among the posterity of Isaac, but that the posterity promised to Abraham would descend neither from Ishmael nor the sons of Keturah, but in the line of Isaac.

This is a strong circumstance insisted on by the apostle, to show the strength of Abraham's faith. It was shown not only by his willingness to offer up the child of his old age—his only son by his beloved wife, but by his readiness, at the command of God, to sacrifice even him on whom the fulfillment of the promises depended.