Avraham as Hashem's shield

במדרש ותתן לי מגן ישעך זה אברהם.

In the Midrash: "And You have given me the shield of Your salvation" (Tehillim 18:36) — this refers to Avraham.

The Midrash identifies the "shield" of Hashem's salvation with Avraham Avinu.

דכתיב אנכי מגן לך.

As it is written, "I am a shield (magen) for you." (Bereishis 15:1)

Hashem promised Avraham that He Himself would be his shield.

פי' הקב"ה הוא עוזר ומושיע ומגן אבל מסר הקב"ה זה הכח לאברהם אע"ה להיות הוא המגן ז"ש אנכי מגן לך.

The explanation: the Holy One is the One who helps, saves, and shields; but the Holy One handed over this power to Avraham Avinu, that he should be the shield — and this is the meaning of "I am a shield for you."

Although all protection comes from Hashem alone, He invested Avraham with this very power, making him a conduit of divine protection. "I am a shield for you" thus also means: I make you into a magen for others.

ולכן הי' כח לאברהם אבינו להציל ולהגין על כל הדור.

Therefore Avraham Avinu had the power to rescue and to shield his entire generation.

Because Hashem entrusted this power to him, Avraham could protect and save all the people of his time.

ז"ש במדרש לך נתתי מגינים של צדיקים:

And this is the meaning of the Midrash: "To you I have given the shields of the tzaddikim."

Avraham was granted the role of "magen," the prototype of the tzaddikim whose merit shields and protects others — a power that originates entirely in Hashem but is channeled through the tzaddik.

Summary: All protection and salvation come from Hashem alone, yet He entrusted this very power to Avraham Avinu — "I am a shield for you" meaning He made Avraham himself into a shield. This is why Avraham could protect his entire generation, and why he is the model of the tzaddikim through whom Hashem's protective power flows to the world.