Parashat Vaeira begins with a surprising statement. God tells Moshe that He has told him something that He had not told the patriarchs, Avraham, Yitzchak, and Yaakov: that His name is Yud Kei Vav Kei. God revealed Himself to the patriarchs with the name Keil Shakkai, but not with the four-letter name. There are two difficulties which arise from these Pesukim. First, it says in Bereishit that God says both to Avraham and to Yaakov the very same declaration made to Moshe, “I am Hashem,” and both Avraham and Yaakov call out in the name of God using the four-letter name. So why does God say that He did not make Himself known to them with this name? Second, does this statement imply that the revelation to Moshe was different or more intimate than those of the patriarchs?
Rashi, based upon the Mechilta DeRashbi, comments that God is saying that the patriarchs did not know Him as One who is faithful to fulfill His commitments, represented by His four-letter name. Since God had promised them that He would redeem their descendants from Egypt but had not fulfilled this commitment during their lifetimes, they did not experience God as the fulfiller of promises. As we know from the Baruch SheAmar prayer, one of God’s attributes is that He is Gozeir UMekayeim, that He decrees and fulfills. This is one of the ways in which God relates to people in this world and how we experience God’s presence. In this regard, the patriarchs did not experience God, since they did not see the ultimate fulfillment of the covenant He made with them.
This answer is broadened by a comment of the Baal HaTurim. When the Pasuk says that God appeared to them as Keil Shakkai, it means that God related to them to promise them a great nation from their numerous children. When God related to them with the four-letter name, it was to promise them the land of Canaan.
Looking through the different revelations of God to the patriarchs, we find that the Baal HaTurim makes an astute observation. Each time that God revealed Himself with the name Keil Shakkai, it was to promise them many children making up a great nation. Regarding Avraham, the verse says, “I am Keil Shakkai, walk before Me and be perfect…and I will increase you plentifully” (Bereishit 17:1-2). By Yaakov, the verse says, “And Keil Shakkai will bless you and make you fruitful and multiply you” (28:3). Both of these citations indicate that the name Keil Shakkai accompanies the promise of children. Each time God says “I am Hashem” with the four-letter name, it is to promise the patriarchs the Land of Canaan. To Avraham, God says, “I am Hashem (four-letter name), who took you out of Ur Kasdim to give you this land as an inheritance” (15:1). And to Yaakov, God says, “I am Hashem (four-letter name)…the land upon which you are lying I will give to you and to your descendants” (28:13). Both times that God says “I am Hashem,” it is to guarantee that the land of Canaan will belong to their descendants.
What emerges from this explanation is that the promise of children was a promise that was fulfilled to the childless Avraham and Sarah, to Yitzchak and Rivkah, and to Yaakov and Rachel. Each received the fulfillment of the promise expressed with Keil Shakkai. The promise of the redemption and settlement in the land of Canaan had not yet been fulfilled, and therefore they did not experience the fulfillment expressed with the four-letter name.
What the Baal HaTurim adds is that while Rashi (based on the Mechilta) says that the patriarchs did not experience the revelation of a particular attribute of God – that He is faithful to fulfill promises – it was not an entire attribute that they didn’t experience. Rather, they didn’t experience the fulfillment of the promise of the land of Canaan expressed using that attribute. God did fulfill other promises made to them, such as the one expressed with the name Keil Shakkai that they would be given children. Although Hashem didn’t fulfill his promises completely to the Avot, by Parashat Vaeira he was ready to completely fulfill His promises to redeem Bnei Yisrael with open miracles.