Musar for Pesachim 234:21

When G–d revealed Himself to Abraham at the ברית בין הבתרים, the Torah reports that He took Abraham "outside," showed him the stars, and challenged him to "count them" (Genesis 15,5). The Zohar (immediately following the part we have quoted previously), states, that this verse means that Abraham was told to free himself from the limitations which his astrological studies up to that point in his life had imposed upon him. When G–d asked him at the beginning of chapter 12 to leave his father's house, the message is that "as long as you adhere to part of your traditional philosophy you will not achieve further philosophical/spiritual success. In order to gain further insights, you must go אל הארץ אשר אראך, to the land I will show you." G–d told him that he would not be able to make this "Aliyah" under his own steam. [Why else would G–d need to show Abraham the way to a land in which he Himself had revealed Himself to Abraham already five years earlier? Ed.] The promise ואעשך לגוי גדול is the reason we say in our daily prayers that G–d is אלוקי אברהם; the word ואברכך is the reason we say אלוקי יצחק; the words ואגדלה שמך, is the reason we recite אלוקי יעקב. The final words in that verse והיה ברכה, are equivalent to a signature, and that is why we conclude the first benediction in our daily עמידה prayer with the words מגן אברהם.

Although all the patriarchs have been described as the מרכבה, they are all included when Abraham is mentioned; he, being the "father" of them all, represents them also. This is the reason that the first benediction in the daily עמידה prayer concludes with the signature מגן אברהם, "shield of Abraham." The term "shield" is used to describe those who support the מרכבה, namely חסד, גבורה, תפארת. This too is the deeper meaning of the words (15, 1) אנכי מגן לך שכרך הרבה מאד, "I am your shield, your reward is very great." Malki Tzedek, king of Shalem, alludes to something similar in Bereshit (14, 19-20): ויברכהו, ויאמר: ברוך אברם לא-ל עליון קונה שמים וארץ וברוך א-ל עליון אשר מגן צריך בידך. "He blessed him saying: Blessed be Abraham of G–d Most High, Owner of Heaven and Earth. And blessed be G–d Most High, who has delivered your foes into your hand." The Zohar on that verse understands this as a reference to the daily עמידה prayer when we say very similar sounding words in the first benediction. If one examines the composition of that benediction closely one can view the words אלוקי אברהם, אלוקי יצחק, אלוקי יעקב as corresponding to the words ברוך אברהם. The words א-ל עליון גומל חסדים טובים correspond to the words וברוך א-ל עליון. The words קונה הכל corresponds to Malki Tzedek's words קונה שמים וארץ. We find similar wording in the repetition of the Friday night עמידה. Finally, the signature of that first benediction of the daily עמידה is מגן אברהם, corresponding to the last words in Malki Tzedek's blessing אשר מגן צריך בידך. It is true that we also encounter the formula in מגן דוד, "shield of David." The difference is that whereas מגן אברהם includes the other carriers of the מרכבה as we have explained, David is the recipient of the foregoing including the parts of the מרכבה that Abraham comprises, seeing that he, David, personifies מלכות. As such David provides the "fourth leg" of that four- cornered מרכבה that we have described as the מרכבה ארג"מן. At any rate, the significance of the promise to Abraham in 12,2 והיה ברכה, "be a blessing," is that בך חותמים, "they will conclude the benediction using your name," is clear. We shall come back later to the meaning of מגן דוד.