Romans 9:7

Nor because they are Abraham’s descendants are they all his children. On the contrary, “Through Isaac your offspring will be reckoned.”

Note Genesis 21:12

BSB · Berean Standard Bible
Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
KJV · King James Version
Neither, because they are Abraham’s offspring, are they all children. But, “your offspring will be accounted as from Isaac.”
WEB · World English Bible
neither, because they are Abraham’s seed, are they all children: but, In Isaac shall thy seed be called.
ASV · American Standard Version

Related passages

From the Treasury of Scripture Knowledge, grouped by the word they explain.

because
Luke 3:8; Luke 16:24, 25, 30; John 8:37–39; Philippians 3:3
In Isaac
Genesis 21:12; Hebrews 11:18