Hebrews 11:18

In Isaac. In view of God’s repeated and emphatic declarations that Isaac was to be the one through whom the covenant promises were to be fulfilled, it was a most extraordinary demonstration of faith on the part of Abraham to be willing to comply with God’s instructions to offer up Isaac as a sacrificial victim. It must have appeared to Abraham that God was on the point of rendering the fulfillment of His promises utterly impossible.

The insertion of this quotation from Genesis 21:12 at this point in the author’s comment on Abraham’s faith explains the sense in which he refers to Isaac as Abraham’s “only begotten” in Hebrews 11:17. Isaac was the only son of Abraham who could qualify as successor to the covenant promises made to Abraham.