וָאֵרָא, אֶל-אַבְרָהָם אֶל-יִצְחָק וְאֶל-יַעֲקֹב--בְּאֵל שַׁדָּי; וּשְׁמִי יְהוָה, לֹא נוֹדַעְתִּי לָהֶם
and I appeared unto Abraham, unto Isaac, and unto Jacob, as El Shaddai, but by My name Hashem I did not reveal to them
The question commonly asked of course is that we see many times when G-d did indeed address Avraham, Yitzchak and Yaakov as Hashem. Furthermore how does G-d answer Moshe by telling him this.
To answer this we need to look at why Moshe is upset. Yes conditions have gotten worse for the Jews, but to our way of looking at it, this is only a temporary phenomenon soon to be relieved by the Exodus. Did Moshe Rabbeinu have so little faith that he had to challenge G-d on this score?
To understand Moshe's problem, we have to remember that he had a different timetable than we do. Moshe's timetable was the one given to Avraham in the Brit Bein Habetarim, which said that the Jews would be in Egypt for 400 years. For all Moshe knew, his duty would be to spend the time remaining of those 400 years, which could be as much as a 190 years, appealing to Pharaoh and trying to inspire faith in the increasingly downtrodden Jews.
While this might seem unlikely to us, keep in mind that Noah had to spend almost as much time building the ark and warning his generation that the flood was coming. And the prospect of his people enduring two centuries of increasing suffering that might destroy their faith entirely, was too much for Moshe.
Now in that light let's look at the answer Moshe received from Hashem. To the Patriarchs, G-d had revealed himself as El Shaddai, setting borders and timelines for the events of the world. As El Shaddai, G-d had said to Avraham Avinu, that the Jews would have to be in Egypt for 400 years. But to Moshe and now to the Jews, G-d revealed himself as Hashem, who went beyond those limits to act out of mercy and compassion.
While Avraham and Yitzchak and Yaakov knew Hashem, they had not seen him transcend boundaries in this way. Moshe and the Jews however now would. Let us now look further at just how this phenomenon was expressed in both eras and how it can be reconciled fully with the covenant G-d made with Avraham at the Brit Bein Habetarim.
This Shabbat was also the Yahrtzeit, the anniversary of the death of Avraham Avinu. This week's Parsha Vaeira, carries an obvious echo in its name of Parshas Vaera, in which G-d appears to Avraham. What happens in Parshas Vaera, word is carried to Avraham by way of three angels that after all these long years, his wife Sarah will finally give birth and give Avraham the child he always wanted. The child that would begin the transmission of what would become the Jewish people. So too Yitzchak and Rivkah would have to pray and wait for their own children to be born. So too Yaakov and Rachel would have to wait a long time for Yosef to be born.
In nature, Sara, Rivka and Rachel could not give birth. It took an extensive amount of waiting and pleading for that to change. Eventually the limit was reached and El Shaddai granted them all children for G-d loves the prayers of the righteous. There was only one exception to this rule, Leah, who had children, quickly and easily.
Why did Leah have children quickly and easily, because as the rejected wife, she was oppressed and in pain already. And so G-d was merciful to her and she had many children, almost without limit. But unlike the Avot, their descendants, the Jews in Israel begin multiplying rapidly, because like Leah, they were suffering and oppressed. But how much of a divine kindness was this really, to create more children at a time when the Egyptians are degrading and oppressing the Jews?
Now let's take a look at the Brit Bein Habetarim again, in which G-d tells Avraham Avinu that the Jews be enslaved for 400 years and that they will return in the 4th generation of Egyptian slavery.
In the attribute of El Shaddai the Jews were destined to be slaves for 400 years. But in the attribute of Hashem, the covenant would still hold so long as it was in the 4th generation. But in response to the Egyptian oppression of the Jews, Hashem made them be fruitful and multiply. While naturally it might indeed have taken 400 years to have 4 generations, due to the oppression, Hashem made the people numerous and 4 generations was reached in barely half that time.
When Moshe arrives before Pharaoh and informs him that Hashem ordered that the Jews be released, Pharaoh replies that he does not know Hashem. As far as he's concerned the deadline is still 400 years. So Pharaoh knows El Shaddai, as a G-d of strict limits, but not one of mercy. And so Moshe accedes, and speaks of Elohei Haivrim. Having forfeited his right to mercy, by rejecting Hashem, Pharaoh must now deal with the absolute power of Elohei Haivrim.
And yet it is Pharaoh's own oppression that caused G-d to accelerate the generations so that the 4th generation is reached far earlier than it would have been under Avraham, Yitzchak and Yaakov. But as it was under Leah in response to her own suffering. That is also why before Moshe addresses Pharaoh, we are interrupted to hear a partial genealogy of Leah's 3 elder sons. 3 of the 4 sons who were born easily and without delay. The partial genealogy lists four generations, beginning with Levi and down to Moshe and Aaron. With Moshe and Aaron, the four generations had been reached, and so it was they who stood before Pharaoh to tell him that by the word of Hashem the time had been reached and the Jews would be set free.