The Talmud Bavli in Berachos (16b) states:
The Rabbis taught: We only refer to the Patriarchs as three and we only
refer to the Matriarchs as four. Why is this true [that there are only
three] Patriarchs? If we would say that we cannot know if we come from
Reuven or if we come from Shimon [i.e. we can only trace our lineage back
with certainty to Avraham, Yitzchok, and Yaakov and not to any one of the
sons of Yaakov], if so then the same [could be said about] the
Matriarchs: we cannot know if we come from Rachel or if we come from
Leah. Rather, [the answer is that] up to this point [i.e. Avraham,
Yitzchok, and Yaakov] they were highly regarded, after this point they
were not [as] highly regarded.
The Talmud here seems to indicate that Avraham, Yitzchok, and Yaakov are
considered Patriarchs, not because of our natural relationship to them,
that they are our ancestors, but because they were great individuals and
"highly regarded" -- *chashivi*. This, we can assume, must be case for
the Matriarchs as well. Sarah, Rivka, Rachel, and Leah are not
Matriarchs because we are descended from them, for we are descended from
Bilha and Zilpa as well, but because of their own personal greatness.
The Talmud is telling us that the three generations of the Patriarchs and
Matriarchs were unique and formed the roots and foundation of the future
nation of Israel. The three Patriarchs and the four Matriarchs were each
instrumental in the spiritual formation of Klal Yisroel. We are all the
children of Rachel and of Leah, just as we are the children of Sarah and
Rivka. The prophet Yirmiyahu said (Jer. 31:14):
A voice was heard in Rama, lamentation, and bitter weeping -- Rachel
weeping for her children; she refused to be comforted for her children,
because they are not.
The prophet is referring to all the people of Israel, not only the
descendants of Rachel. We are all her children, as we are all the
children of each of the Matriarchs and Patriarchs. Though Yaakov had
four wives, and together they physically mothered the twelve Tribes, only
Rachel and Leah are considered Matriarchs. It is very apparent from
reading the Torah that Rachel and Leah were the movers and doers in the
formation of Yaakov's family. Bilha and Zilpa were introduced into the
family only for Rachel and Leah's sake and by their bidding. Rachel said
to Yaakov (Gen. 30:3):
Behold my maid Bilha, go in to her, and she shall bear upon my knees,
that I may also have children through her.
And so it was with all the children of Bilha and Zilpa. In fact, after
bearing their children, Bilha and Zilpa seem to disappear from the text.
Their purpose was completed -- to provide children for Rachel and Leah.
Rachel and Leah took those children, and together with their own, forged
the nation of Israel. They, with Sarah and Rivka, were the Matriarchs of