Parashat Vayishlach concludes with a presentation of Esav's lineage. This section begins by listing his wives, which include two Hittite women, Ada and Ahalivama (36:2). Rashi identifies Ahalivama as Yehudit, whom we met earlier, in Parashat Toledot, as one of Esav's Hittite wives (26:34). Rashi explains the discrepancy in her name by claiming that her real name was Ahalivama. Esav changed her name to Yehudit in order to mislead his parents into thinking that she rejected idolatry. Apparently, "Yehudit," which of course means "Jew," symbolizes the rejection of paganism. Obviously, this presents a serious problem. We call ourselves "Yehudim," Jews, because we descend from the Southern Kingdom that was led by the tribe of Yehuda, whose territory was thus named Judea. Now when Esav married Ahalivama, Yaakov had not even married yet, let alone begotten his fourth son, Yehuda. How, then, did the name "Yehudit" signify the opposition to paganism? (See Rav Yaakov Kaminetzky, in his "Emet Le-Yaakov.)
We have no choice but to explain that this term bore significance even before the birth of Yehuda. To identify the source of this significance, we need only to note the origin of Yehuda's name. Upon his birth, his mother, Leah, declares, "This time I will thank God" ("ha-pa'am odeh at Hashem" - 29:35). The name Yehuda thus means the expression of gratitude, or the recognition of the Almighty as solely responsible for evin the universe. From Rashi it clearly emerges that this very point lies at the heart of monotheism and marks the fundamental distinction between it and idolatry.
We may perhaps gain a clearer understanding of this point by considering the Gemara's comment in Masekhet Berakhot 7b. The Gemara there claims that no one ever thanked the Almighty until Leah did upon Yehuda's birth. Many commentators have struggled to understand the Gemara's intent. Could it be that no one ever expressed gratitude to God before Leah delivered her fourth child? On one level, this Gemara will always remain an enigma: it's hard to imagine that Leah introduced a new dimension of gratitude that Avraham, Yitzchak or Yaakov never expressed. Nevertheless, attempts have been made to identify this new dimension of Leah's thanksgiving. Some have claimed that whereas many of her predecessors thanked God for supernatural or extraordinary events, Leah represented a constant acknowledgment of God's kindness, as manifest even in ordinary occurrences. We may add that at all her other births, Leah issued a declaration explicitly referring to her status vis-à-vis Yaakov, she was thankful specifically for the "promotion" she earned as a result of her having borne children. This time, she makes no mention whatsoever of her husband's affection for her; she states simply, "This time I will thank God." It appears as though at this point Leah achieves a generic sense of appreciation for God's goodness, irrespective of any specific acts of kindness.