Musar על פסחים 234:11

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.