At the beginning of Parshat Vaera, God tells Moshe that His name is Hashem – the four letter name that is unpronounceable. God then says that He never revealed this name to Abraham, Isaac, and Jacob. Why not? Were they not worthy enough to hear that name?
Rashi explains that the “name” represents absolute truth. Since God did not fulfill His promises to the Patriarchs (to build them into a great nation and give them the land of Israel) in their lifetime, He could not represent Himself with that name. Since the Jews would be redeemed from Egypt by Moses, God could finally use the name of absolute truth.
I’d like to offer another explanation. According to the mystical teachings, the four letter name of God represents Chesed-Giving, or Rachamim-Mercy. The Patriarchs had hard lives. Their wives suffered years of barrenness. They faced persecution and pursuers, enemies and rivalries. They dealt with family strife, rebellious children, and hostile relatives. They were constantly being tested by God. They were only shown the Din-Judgment representation of God, not the Chesed-Rachamim-Mercy aspect, as represented in the four letter name. The Jews in Egypt, however, could only see the name of mercy. They did not deserve redemption on the basis of their own merit. Their redemption was solely based on the Chesed-Mercy of God, as represented in His four letter name.