The 16 expressions of forgiveness

The Magen David as an expression of divinity in the world.

The righteous have "their name" mentioned before them, "his name was Boaz", "his name was Mordochai." While the wicked have "their name" after them, "Golyat was his name", "Naval was his name." The reason Tzadikim have "their name before them" is that they are compared to their creator, "..and my name Hashem was not known to them." 

So we see Hashem's promise to David is, "I will make for you a name like the name of the great ones on the earth." Our sages say, in Pesachim, that this refers to the fact that we say after haftorah the brocha, "magen David." The "great ones" refers to the Avos, like we say, "magen Avraham."

The Pardes brings a proof for this from the Zohar that "magen" refers to the level of "bris", yesod. The source is from the verse "a son and it's sheath are Hashem E-lokim." It's a concept of a supernal "union" of the masculine and feminine concepts. A yichud via the "bris." By tzadikim this refers to the service of "mekadshei shmecha," that they "sanctify" Hashem's name. The draw down from the level of "ata kadosh", "you are holy", to "shimcha kadosh," "you're name is holy." 

Further, "magen" is made up of the letters for, the malachim "Michoel, Gavriel and Nuriel," the three "camps" of the Shchina. It's the idea of bringing down this sanctification of Hashem's name down in to the lower realms.

This is understood according to the explanation we learn from the Kuzari of why we even have such a concept of "E-lokei Avraham." Why should Hashem's name be associated with a person? Wouldn't it be better to associate Hashem's name with the creation in general? The answer is that it was Avraham who spread the awareness of Hashem in the world, therefore it's called in his name. Avraham "guarded (shielded, if you will)" and was the advocate ofthe expression of divinity within creation. Thus the "magen," "shield" is associated with the covenant between creation and the divine, the "bris," the flow of yesod.

So thus Rashi explains the verse, "all who are called in my name, I created them, I formed them and furthermore have made them." "Those who are called in my name" are the tzadikim who are called in the name of Hashem." Since they are the ones who draw it down.

Ohr Hatora second Veleh Shemos