Guardian of the Doors of Israel
“King Ardavan IV of Parthia sent a pearl of purest radiance to Rabbi [Yehudah the Prince] and requested: Send me something equally precious in return. Rabbi sent him a mezuzah. The king sent back word: I sent you something beyond price, and you sent me something that sells for a debased coin of no value! Rabbi replied: My desirable things and your desirable things are not to be compared. You sent me something that I have to guard; I sent you something that guards while you lie asleep, as it is said: When you walk about, it will guide you, when you lie down, it will guard you… (Proverbs 6:22)” (JT Peah 1).
“[When] Onkelos the son of Kalonymus became a proselyte, the Emperor sent a contingent of Roman [soldiers] after him but he enticed them by [citing] scriptural verses and they became converted to Judaism… So they took hold of him; and as they were walking on he saw the mezuzah which was fixed on the door-frame and he placed his hand on it saying to them: ‘Now what is this?’ and they replied: ‘You tell us then.’ Said he, ‘According to universal custom, the mortal king dwells within, and his servants keep guard on him without; but [in the case of] the blessed Holy One it is His servants who dwell within whilst He keeps guard on them from without; as it is said: YHWH guards your going and coming, now and for evermore (Psalms 121:8). Then they, too, were converted to Judaism. [The Emperor] he sent for him no more” (BT Avodah Zarah 11a).
“Shekhinah is the gate onto which the mezuzah is affixed: זֶה (Zeh), This, is the gate of YHWH (Psalms 118:20)” (Zohar 3:258a, Ra’aya Meheimna Pineḥas).