"ברוך אתה ... אלקי אברהם אלקי יצחק ואלקי יעקב ... ברוך אתה ה' מגן אברהם"
“Blessed are You, G‑d of Avraham, G‑d of Yitzchak, and G‑d of Yaakov. Blessed are You, G‑d, shield of Avraham.”

QUESTION: Why does the berachah start with mentioning our three patriarchs and conclude only with Avraham?

ANSWER: On the pasuk “I will make you into a big nation: I will bless you, and make you famous and you will be a blessing” (Bereishit 12:2), Rashi explains that this refers to what we say in Shemoneh Esreih, “Elokei Avraham, Elokei Yitzchak, Elokei Yaakov,” “but,” Hashem told Avraham, “the berachah will be concluded with your name only — magen Avraham.” Wouldn’t Avraham be happier if Yitzchak and Yaakov were also mentioned in the conclusion of the berachah?

In Pirkei Avot (1:2) we learn that the world stands on three pillars: 1) The study of Torah, 2) avodah — the service of G‑d, and 3) gemilut chassadim — acts of kindness, tzedakah. The patriarchs each epitomize one of these pillars. Avraham represents chessed (21:33), Yitzchak — avodah (24:63), and Yaakov — Torah (25:27).

According to Rashi, the pasuk is projecting the history of Klal Yisrael. There will be a time when the major relationship between the Jews and Hashem will be through the study of Torah (Elokei Yaakov). At other times it will be through the service in the Beit Hamikdash or through tefillah — prayer (Elokei Yitzchak), and there will be a period when it will be through chessed — tzedakah (Elokei Avraham).

However, the “concluding phase” of galut and the coming of Mashiach will not be dependent on all three pillars, but in the zechut of chessed alone, which is personified by Avraham.

(שמעתי מזקני הרב צבי הכהן ז"ל קפלן)