Take a look at the verses in Genesis dealing with the birth of the twins Esau (who was born first) and Jacob (who, being born immediately afterward, was not the firstborn heir).
21 And Isaac prayed to the LORD for his wife, because she was barren. And the LORD granted his prayer, and Rebekah his wife conceived.
22 The children struggled together within her, and she said, "If it is thus, why is this happening to me?"[2] So she went to inquire of the LORD.
23 And the LORD said to her,
"Two nations are in your womb, and two peoples from within you[3] shall be divided; the one shall be stronger than the other, the older shall serve the younger."
As you may know, Jacob eventually gained the position of heir to Isaac; Esau was compensated, but did not inherit. So according to the Hebrew Bible itself, Abraham had offspring who did not become part of the people of Israel. Belonging to "Israel," Paul argues, is thus not a matter of biological descent. The implications of this are spelled out in Romans 9: 25-33, just ahead.