Hebrews 11:18

and had been told that it is through Isaac that your offspring shall be reckoned ??19 for he considered that God was able even to raise men from the dead. Hence he did get him back, by what was a parable of the resurrection.

Romans 9:7-8

they are not all children of Abraham because they are descended from Abraham. No, it is through Isaac that your offspring shall be reckoned ??8 meaning that instead of God's children being the children born to him by natural descent, it is the children of the Promise who are reckoned as his true offspring.

Treasury of Scripture Knowledge

Summary

And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.

Bible References

In isaac

Romans 9:7
they are not all children of Abraham because they are descended from Abraham. No, it is through Isaac that your offspring shall be reckoned ??8 meaning that instead of God's children being the children born to him by natural descent, it is the children of the Promise who are reckoned as his true offspring.
Hebrews 11:18
and had been told that it is through Isaac that your offspring shall be reckoned ??19 for he considered that God was able even to raise men from the dead. Hence he did get him back, by what was a parable of the resurrection.