(ד) מִנְהָג פָּשׁוּט שֶׁכּוֹתְבִים עַל הַמְּזוּזָה מִבַּחוּץ כְּנֶגֶד הָרֶוַח שֶׁבֵּין פָּרָשָׁה לְפָרָשָׁה שַׁדַּי וְאֵין בָּזֶה הֶפְסֵד לְפִי שֶׁהוּא מִבַּחוּץ. אֲבָל אֵלּוּ שֶׁכּוֹתְבִין מִבִּפְנִים שְׁמוֹת הַמַּלְאָכִים אוֹ שֵׁמוֹת קְדוֹשִׁים אוֹ פָּסוּק אוֹ חוֹתָמוֹת הֲרֵי הֵן בִּכְלַל מִי שֶׁאֵין לָהֶם חֵלֶק לָעוֹלָם הַבָּא. שֶׁאֵלּוּ הַטִּפְּשִׁים לֹא דַּי לָהֶם שֶׁבִּטְּלוּ הַמִּצְוָה אֶלָּא שֶׁעָשׂוּ מִצְוָה גְּדוֹלָה שֶׁהִיא יִחוּד הַשֵּׁם שֶׁל הַקָּדוֹשׁ בָּרוּךְ הוּא וְאַהֲבָתוֹ וַעֲבוֹדָתוֹ כְּאִלּוּ הוּא קָמֵעַ שֶׁל הֲנָיַת עַצְמָן כְּמוֹ שֶׁעָלָה עַל לִבָּם הַסָּכָל שֶׁזֶּהוּ דָּבָר הַמְהַנֶּה בְּהַבְלֵי הָעוֹלָם:
(4) It is a universal custom to write the word Shakkai (Almighty) on the other side of the Mezuzah, opposite the blank space between the two sections. As this word is written on the outside, the practice is unobjectionable. They, however, who write names of angels, holy names, a Biblical text or inscriptions usual on seals, within the Mezuzah, are among those who have no portion in the world to come. For these fools not only fail to fulfill the commandment but they treat an important precept that expresses the Unity of God, the love of Him, and His worship, as if it were an amulet to promote their own personal interests; for, according to their foolish minds, the Mezuzah is something that will secure for them advantage in the vanities of the world.
§ The Gemara mentions other Romans who converted to Judaism. It relates: Onkelos bar Kelonimos converted to Judaism. The Roman emperor sent a troop [gunda] of Roman soldiers after him to seize Onkelos and bring him to the emperor. Onkelos drew them toward him with verses that he cited and learned with them, and they converted... The emperor then sent another troop of soldiers after him, to bring Onkelos, and said to them: Do not converse with him at all. The troops followed this instruction, and took Onkelos with them. While they grabbed him and were walking, Onkelos saw a mezuza that was placed on the doorway. He placed his hand upon it and said to the soldiers: What is this? They said to him: You tell us. Onkelos said to them: The standard practice throughout the world is that a king of flesh and blood sits inside his palace, and his servants stand guard, protecting him outside; but with regard to the Holy One, Blessed be He, His servants, the Jewish people, sit inside their homes and He guards over them outside. As it is stated: “The Lord shall guard your going out and your coming in, from now and forever” (Psalms 121:8). Upon hearing this, those soldiers also converted to Judaism. After that, the emperor sent no more soldiers after him.
(א) בכלבו מה שנהגו לכתוב זה השם יותר משאר שמות מפני ששם זה נוטריקון שומר דירת ישראל...
The Kol Bo explains that the practice to specifically write the name Shakkai, as opposed to any other Divine name, [on the outside of the mezuzah] is due to the fact that the letters [shin, dalet, and yud] which spell the aforementioned name can be seen as an acronym for Shomer Diras Yisrael (the Guardian of the dwellings of Israel)...
(1) O you who dwell in the shelter of the Most High and abide in the protection of Shakkai— (2) I say of the LORD, my refuge and stronghold, my God in whom I trust, (3) that He will save you from the fowler’s trap, from the destructive plague. (4) He will cover you with His pinions; you will find refuge under His wings; His fidelity is an encircling shield. (5) You need not fear the terror by night, or the arrow that flies by day, (6) the plague that stalks in the darkness, or the scourge that ravages at noon. (7) A thousand may fall at your left side, ten thousand at your right, but it shall not reach you. (8) You will see it with your eyes, you will witness the punishment of the wicked. (9) Because you took the LORD—my refuge, the Most High—as your haven, (10) no harm will befall you, no disease touch your tent. (11) For He will order His angels to guard you wherever you go. (12) They will carry you in their hands lest you hurt your foot on a stone. (13) You will tread on cubs and vipers; you will trample lions and cobras. (14) “Because he is devoted to Me I will deliver him; I will keep him safe, for he knows My name. (15) When he calls on Me, I will answer him; I will be with him in distress; I will rescue him and make him honored; (16) I will let him live to a ripe old age, and show him My salvation.”
(יג) חַיָּב אָדָם לְהִזָּהֵר בִּמְזוּזָה מִפְּנֵי שֶׁהִיא חוֹבַת הַכּל תָּמִיד. וְכָל זְמַן שֶׁיִּכָּנֵס וְיֵצֵא יִפְגַּע בְּיִחוּד הַשֵּׁם שְׁמוֹ שֶׁל הַקָּדוֹשׁ בָּרוּךְ הוּא וְיִזְכֹּר אַהֲבָתוֹ וְיֵעוֹר מִשְּׁנָתוֹ וְשִׁגְיוֹתָיו בְּהַבְלֵי הַזְּמַן. וְיֵדַע שֶׁאֵין דָּבָר הָעוֹמֵד לְעוֹלָם וּלְעוֹלְמֵי עוֹלָמִים אֶלָּא יְדִיעַת צוּר הָעוֹלָם. וּמִיָּד הוּא חוֹזֵר לְדַעְתּוֹ וְהוֹלֵךְ בְּדַרְכֵי מֵישָׁרִים. אָמְרוּ חֲכָמִים הָרִאשׁוֹנִים כָּל מִי שֶׁיֵּשׁ לוֹ תְּפִלִּין בְּרֹאשׁוֹ וּבִזְרוֹעוֹ וְצִיצִית בְּבִגְדוֹ וּמְזוּזָה בְּפִתְחוֹ מֻחְזָק הוּא שֶׁלֹּא יֶחֱטָא שֶׁהֲרֵי יֵשׁ לוֹ מַזְכִּירִין רַבִּים וְהֵן הֵם הַמַּלְאָכִים שֶׁמַּצִּילִין אוֹתוֹ מִלַּחֲטֹא שֶׁנֶּאֱמַר (תהילים לד ח) ״חֹנֶה מַלְאַךְ יְיָ׳ סָבִיב לִירֵאָיו וַיְחַלְּצֵם״.
בְּרִיךְ רַחֲמָנָא דְּסַיְּעָן:
(13) A person should pay heed to the precept of the Mezuzah; for it is an obligation perpetually binding upon all. Whenever one enters or leaves a home with the Mezuzah on the doorpost, he will be confronted with the Declaration of God's Unity, blessed be His holy name; and will remember the love due to God, and will be aroused from his slumbers and his foolish absorption in temporal vanities. He will realize that nothing endures to all eternity save knowledge of the Ruler of the Universe. This thought will immediately restore him to his right senses and he will walk in the paths of righteousness.
(ד) אַף עַל פִּי שֶׁתְּקִיעַת שׁוֹפָר בְּרֹאשׁ הַשָּׁנָה גְּזֵרַת הַכָּתוּב רֶמֶז יֵשׁ בּוֹ כְּלוֹמַר עוּרוּ יְשֵׁנִים מִשְּׁנַתְכֶם וְנִרְדָּמִים הָקִיצוּ מִתַּרְדֵּמַתְכֶם וְחַפְּשׂוּ בְּמַעֲשֵׂיכֶם וְחִזְרוּ בִּתְשׁוּבָה וְזִכְרוּ בּוֹרַאֲכֶם. אֵלּוּ הַשּׁוֹכְחִים אֶת הָאֱמֶת בְּהַבְלֵי הַזְּמַן וְשׁוֹגִים כָּל שְׁנָתָם בְּהֶבֶל וָרִיק אֲשֶׁר לֹא יוֹעִיל וְלֹא יַצִּיל, הַבִּיטוּ לְנַפְשׁוֹתֵיכֶם וְהֵיטִיבוּ דַּרְכֵיכֶם וּמַעַלְלֵיכֶם וְיַעֲזֹב כָּל אֶחָד מִכֶּם דַּרְכּוֹ הָרָעָה וּמַחֲשַׁבְתּוֹ אֲשֶׁר לֹא טוֹבָה. לְפִיכָךְ צָרִיךְ כָּל אָדָם שֶׁיִּרְאֶה עַצְמוֹ כָּל הַשָּׁנָה כֻּלָּהּ כְּאִלּוּ חֶצְיוֹ זַכַּאי וְחֶצְיוֹ חַיָּב. וְכֵן כָּל הָעוֹלָם חֶצְיוֹ זַכַּאי וְחֶצְיוֹ חַיָּב. חָטָא חֵטְא אֶחָד הֲרֵי הִכְרִיעַ אֶת עַצְמוֹ וְאֶת כָּל הָעוֹלָם כֻּלּוֹ לְכַף חוֹבָה וְגָרַם לוֹ הַשְׁחָתָה. עָשָׂה מִצְוָה אַחַת הֲרֵי הִכְרִיעַ אֶת עַצְמוֹ וְאֶת כָּל הָעוֹלָם כֻּלּוֹ לְכַף זְכוּת וְגָרַם לוֹ וְלָהֶם תְּשׁוּעָה וְהַצָּלָה שֶׁנֶּאֱמַר (משלי י כה) "וְצַדִּיק יְסוֹד עוֹלָם" זֶה שֶׁצָּדַק הִכְרִיעַ אֶת כָּל הָעוֹלָם לִזְכוּת וְהִצִּילוֹ. וּמִפְּנֵי עִנְיָן זֶה נָהֲגוּ כָּל בֵּית יִשְׂרָאֵל לְהַרְבּוֹת בִּצְדָקָה וּבְמַעֲשִׂים טוֹבִים וְלַעֲסֹק בְּמִצְוֹת מֵרֹאשׁ הַשָּׁנָה וְעַד יוֹם הַכִּפּוּרִים יֶתֶר מִכָּל הַשָּׁנָה. וְנָהֲגוּ כֻּלָּם לָקוּם בַּלַּיְלָה בַּעֲשָׂרָה יָמִים אֵלּוּ וּלְהִתְפַּלֵּל בְּבָתֵּי כְּנֵסִיּוֹת בְּדִבְרֵי תַּחֲנוּנִים וּבְכִבּוּשִׁין עַד שֶׁיֵּאוֹר הַיּוֹם:
(4) Notwithstanding that the blowing of the ram's horn trumpet on Rosh ha-Shanah is a Scriptural statute, its blast is symbolic, as if saying: "Ye that sleep, bestir yourselves from your sleep, and ye slumbering, emerge from your slumber, examine your conduct, turn in repentance, and remember your Creater! They that forget the truth because of the vanities of the times, who err all of their years by pursuing vanity and idleness, which are of neither benefit nor of salvation, care for your souls, improve your ways and your tendencies, let each one of you abandon his evil path and his thought which is not pure! It is, therefore, necessary for every man to behold himself throughout the whole year in a light of being evenly balanced between innocence and guilt, and look upon the entire world as if evenly balanced between innocence and guilt; thus, if he commit one sin, he will overbalance himself and the whole world to the side of guilt, and be a cause of its destruction; but if he perform one duty, behold, he will overbalance himself and the whole world to the side of virtue, and bring about his own and their salvation and escape, even as it is said: "But the righteous is an everlasting foundation" (Prov. 10. 25), it is he, by whose righteousness he overbalanced the whole world to virtue and saved it...
(5) And the officers shall speak with the people, saying: Who is the man who has built a new house, but not yet fixed mezuzot in it to complete it? let him go and return to his house, lest through sin he be slain in the battle, and another man complete it.