Rabbi Chaim Attar, in his commentary Or Hachaim, asks why G-d felt it necessary to make this declaration to Moshe, when he already told him, "This is my name, that is Hashem, forever," at the very beginning of his mission to redeem the Jews from Egypt.
The Or Hachaim’s answer presents us with a lesson of faith. At the end of Parshas Shemos, Moshe and Aaron come to Pharaoh to plead their case on behalf of the Israelites. The king of Egypt rebuked them and not only continued the Jews’ enslavement and persecution, but worsened their conditions.
The Israelites responded by complaining to Moshe and Aaron, who brought their plea before G-d. "Why have you done evil to this people? Why have you sent me?" Moshe asks.
G-d’s answer is our verse in question at the beginning of Va’eira. Elokim, representing G-d’s attribute of judgment (midas hadin) spoke to Moshe and said to him "I am Hashem"—the name that encompasses G-d’s attribute of mercy and compassion (midas harachamim).
The inadequate human mind often sees G-d as Elokim, as a harsh G-d. But the reality is that everything G-d does is encapsulated in His midas harachamim, "I am Hashem—I am the G-d of mercy."
Although Moshe seems to experience G-d through His midas hadin, in reality G-d’s midas harachamim is being demonstrated, for "Hashem hu haElokim."
In his essay, "Kol Dodl Dofeik," Rav Yosef D. Soloveitchik, zt"l, sums up this lesson in a most eloquent passage.
"Man’s apprehension is limited and distorted.... As long as he perceives only isolated fragments of the cosmic drama and the mighty epic of history, he remains unable to penetrate into the secret lair of suffering and evil. To what may the matter be compared? To a person gazing at a beautiful tapestry, a true work of art, one unto which an exquisite design has been woven, but looking at it from the reverse side. Can such a viewing give rise to a sublime aesthetic experience? We, alas, view the world from its reverse side. We are unable to grasp the all encompassing framework of being. And it is only within that framework that is possible to discern the divine plan, the essential nature of the divine actions."
Let us assimilate the lesson of faith that we draw from G-d’s encounter with Moshe Rabbeinu and the Jewish people as they experience redemption from Egypt.