According to Jewish custom, the line of descent is matrilineal, that is, if your mother is a Jew, then you are one too.

That being said, why, in the book of Genesis, is it promised to Abraham that his descendents shall be as numerous as the stars of the sky? And why does the Nation of Israel, later in the Tanach , consider themselves the descendants of Abraham? According to Jewish custom, they would be the descendants of Sarai, Rebecca, Rachel and Leah. In other words, why would thepatriarchs of the Hebrews be important in a matrilineal society?

A good answer for this question could be either from within Jewish custom, or it could be the answer is within a historical perspective, that is, that the Jewish people didn't adopt matrilineal descent until far after the time of the Tanach. (That is, either story internal or story external)