Rav Yisrael Salanter teaches that yirat shamayim is not achieved by intellectual knowledge alone but requires the emotional and psychological transformation that comes through Mussar study done with intensity (hitpa'alut), repetition, and niggun.
אָכֵן לְמוֹרַת רוּחַ וְדַאֲבוֹן לֵב, כֹּחַ הַכְּלָלִי הַזֶּה, רַק מִסְתַּתֵּר בָּנוּ, נֶחְבָּא בְּמַצְפּוּנֵי הַלֵּב בַּל יִרְאֶה הַחוּצָה, אִם לֹא נָשִׂים לֵב לְשַׁדֵּד אַדְמַת לְבָבֵנוּ בְּהַרְחָבַת רַעְיוֹנֵי הַמּוּסָר. אִי לָזאֹת גַּם הַכֹּחַ הַכְּלָלִי הַלָּזֶה בַּל יִשְׁלַח פֹּארוֹתָיו עַל הָאֵבָרִים לְאָסְרָם בְּמַאֲסַר הַיִּרְאָה. וּמִזֶּה הַכְּלָל לֹא יִמָּשְׁכוּ הַפְּרָטִים, לְהִשָּׁמֵר מֵהָעֲבֵרוֹת הַמִּתְרַגְּשׁוֹת. וְנִכְשָׁלִים אֲנַחְנוּ בְּכָל עֵת בַּעֲבֵרוֹת רָמוֹת עַד שָׁמַיִם יַגִּיעוּ, בְּחֶטְאֵי הַלָּשׁוֹן בְּאֵין מַעֲצוֹר לְרוּחֵנוּ, וּבְעִסְקֵי מַשָּׂא וּמַתָּן לְמַכְבִּיר, וּבִטּוּל תּוֹרָה עַל כֻּלָּנָה. וּבִכְלָל, בְּכָל הָאֵבָרִים כִּמְעַט אֵין בָּמוֹ מְתוֹם, לַמִּתְבּוֹנֵן הֵיטֵב אִישׁ לְפִי עֶרְכּוֹ, שֶׁכָּל הַגָּדוֹל מֵחֲבֵרוֹ יִצְרוֹ גָּדוֹל מִמֶּנּוּ (סֻכָּה נ"ב), וְנִכְשָׁל בַּעֲבֵרוֹת נִשָּׂאוֹת וְרָמוֹת, אֲשֶׁר אִם גַּם לְחֶשְׁכַת לְבָבֵנוּ כְּסוּמִים נֶחְשַׁבְנוּ, אֵין רוֹאֶה גָּדְלָם (נֶגֶד עֲבֵרוֹת הֶהָמוֹן). רַק אִם נַשְׁקִיפֵם עַל יְדֵי כְּלֵי הַמַּחֲזֶה (הַמַגְדֶּלֶת דָּבָר גָּדוֹל לִמְאֹד, הַנִּרְאֶה לְחֻלְשַׁת רְאוּתֵנוּ כְּקָטָן, כְּמוֹ הַכּוֹכָבִים אֲשֶׁר גָּדְלָם יוֹתֵר מִכַּדּוּר הָאָרֶץ, וְנִרְאִים כִּנְקֻדּוֹת קְטַנּוֹת, וְעַל יְדֵי כְּלֵי הַמַּחֲזֶה נִרְאִים יוֹתֵר גְּדוֹלִים מְעַט, וּבֶאֱמֶת אֵין עֲרֹךְ לְגָדְלָם. כֵּן הַדָּבָר הַזֶּה), הוּא הַשֵּׂכֶל הַנָּכוֹן, עַל פִּי הַתּוֹרָה הַנֶּאֱמָנָה, נִמְצָא אוֹתָם חֲמוּרִים הַרְבֵּה בְּאֵיכוּתָם בְּיֶתֶר שְׂאֵת וְיֶתֶר עֹז: נָשִׂימָה לֵב לְהִתְבּוֹנֵן בְּעִנְיַן הָעֲבֵרוֹת, וְנִרְאֶה כִּי שְׁתַּיִם הֵנָּה בְּסוּגֵיהֶם: הָאַחַת, נוֹבַעַת מֵהַתַּאֲוָה הַנִּפְרָצָה, לֶאֱהֹב הֶעָרֵב לִשְׁעָתוֹ, לִבְלִי הַשְׁקִיף הַנּוֹלָד, אִם כִּי מָרָה תִּהְיֶה בָּאַחֲרוֹנָה. נִמְצָא דּוּגְמָתָהּ מַה גַּם בְּעִנְיְנֵי הָעוֹלָם, הָאִישׁ הַסָּכָל, וּבִפְרָט הַחוֹלֶה לְחֻלְשַׁת שִׂכְלוֹ, יֶאֱהַב לַחֲטֹף לֶאֱכֹל אֶת הֶעָרֵב לְחִכּוֹ, וְיִשְׁכַּח כִּי זֶה יְבִיאֶנּוּ לְמַחֲלָה נִפְרָצָה. לְכָךְ אָמְרוּ רַבּוֹתֵינוּ זַ"ל (אָבוֹת) אֵיזֶה חָכָם הָרוֹאֶה אֶת הַנּוֹלָד, הוּא מַאֲמַר רַבּוֹתֵינוּ זַ"ל (סוֹטָה ג') אֵין אָדָם עוֹבֵר עֲבֵרָה אֶלָּא אִם כֵּן נִכְנְסָה בּוֹ רוּחַ שְׁטוּת. וְזֶה כָּל עֲמַל הָאָדָם לַעֲבוֹדָתוֹ יִתְבָּרַךְ, לַחֲשֹׁב וּלְהִתְבּוֹנֵן בְּיִרְאַת ה' בְּפַחַד עָנְשׁוֹ עַל יְדֵי סִפְרֵי הַמּוּסָר וְאַגָּדוֹת חֲכָמֵינוּ זַ"ל, עַד אֲשֶׁר בְּאָזְנָיו יִשְׁמַע כִּמְעַט בְּעֵינָיו יִרְאֶה הָעֹנֶשׁ הַגָּדוֹל בְּכַמּוּת וְאֵיכוּת נִצָּב לְנֶגֶד עֵינָיו, כְּמַאֲמַר רַבּוֹתֵינוּ זַ"ל (סַנְהֶדְרִין ז') לְעוֹלָם יִרְאֶה דַּיָּן, (לִהְיוֹתוֹ עָלוּל לְהִכָּשֵׁל, וְהוּא הַדִּין כָּל אָדָם. לְהִשָּׁמֵר מִפֶּגַע רַע מֵעֲבֵרוֹת הַחֲמוּרוֹת) כְּאִלּוּ כוּ' גֵּיהִנֹּם פְּתוּחָה לוֹ מִתַּחְתָּיו. וְאִם כֹּה יַעֲשֶׂה הָאָדָם וּלְבָבוֹ יָבִין, וְשָׁב וְרָפָא לוֹ: אָכֵן רַבָּה רָעַת הָאָדָם בָּאָרֶץ, אֵין דּוֹרֵשׁ בְּצֶדֶק וְאֵין מַשְׂכִּיל בְּיִרְאַת ה', לִקְבֹּעַ עִתִּים סְדוּרוֹת בַּעֲמַל הַיִּרְאָה, לִדְלוֹת מֵי תְּבוּנָה מֵהָאֱמוּנָה הַנֶּחְבָּאָה וּמְסֻתֶּרֶת בְּמַחֲבוֹאֵי הַלֵּב, לְהַרְחִיבָהּ וּלְסַעֲדָהּ, לָתֵת לָהּ תֹּקֶף וָעֹז, לָשִׂים הַמִּשְׂרָה עַל שִׁכְמָהּ, לִמְשֹׁל עַל הָאֵבָרִים לְבַל יִפְרְצוּ גִּדְרָהּ וְכַתּוֹרָה יַעֲשׂוּ. זֶהוּ הַסּוּג הַשֵּׁנִי. דּוּגְמַת זֶה הַדָּבָר לֹא מָצָאנוּ בְּעִנְיְנֵי הָעוֹלָם, אֵין אִישׁ שֶׁצָּרוֹת מְרַחֲפוֹת מִמּוּל פָּנָיו, אֲשֶׁר לֹא יִקַּח לוֹ עֵת לַחֲשֹׁב מַחֲשָׁבוֹת לְהִנָּצֵל מֵהֶן. אִי לָזֹאת, גַּם אֵין הַתַּאֲוָה סִבָּתָהּ, פְּלִיאָה הִיא הָעֲבֵרָה הַלָּזוֹ, וְזֶה שְׁמָהּ פִּלְאִי, לִבְלִי שׂוּם עַל לֵב לְהִתְבּוֹנֵן בְּיִרְאַת ה' וּבְמוּסַר עָנְשׁוֹ, עֲבֵרוֹת כָּאֵלּוּ לֹא יוּכְלוּ הֱיוֹת מִסּוּג הָרִאשׁוֹן, יַעַן אֵין דַּי בְּהַתַּאֲוָה לִהְיוֹת סִבָּתָן, רַק הֵן מֵרוּחָנִיּוּת הַטֻּמְאָה הַמְלַפֶּפֶת אֶת הָאָדָם לְהַחֲטִיאוֹ, וּבְיוֹתֵר בְּחֶטְאֵי הַדִּבּוּר, כִּי מַה יִּתְרוֹן לְבַעַל הַלָּשׁוֹן, כְּמַאֲמַר רַבּוֹתֵינוּ זַ"ל (עֲרָכִין ט"ו) עַיֵּן שָׁם:
Truly, to our chagrin and regret, this universal force [i.e., recognition that there is ultimate judgment,] is only latent within us, concealed in the inner recesses of the heart and not outwardly visible – unless we take care to till the soil of our heart by expanding on the concepts of Mussar. Without this, even this universal force will not extend its tendrils to the limbs, to bind them within the grasp of reverence. From this general principle, no specific ramifications will be drawn to safeguard against looming sins. So we constantly stumble into sins that tower unto the heavens: with sins of speech in the absence of a brake on our spirit; and even to a greater extent in business matters; and most of all, in neglect of Torah study. In general, almost in all the limbs, “[from head to toe] there is not a [morally] sound [spot],” [according] to one who analyzes carefully, each according to his own ability, for “one who is greater than his friend has a greater evil urge than him” (Sukka 52a) and falls into high and mighty sins. When it comes to seeing the darkness in our own heart, we are like blind people. No one sees their magnitude (compared to the sins of the masses). Only if we observe them through a telescope (which magnifies to a very large degree things that seem small to us due to our weak sight – like stars, which are larger than Earth but seem like small dots; through a telescope they appear a bit larger – but in truth, they are inestimably large. So it is in this matter) – that is, with correct reason, in accordance the trusted Torah – will we find them to be far more severe in character, with greater scope and force. Let us pay attention and analyze the matter of sins. We will see that there are two types. One stems from unbridled lust, loving what is immediately gratifying without paying heed to the outcome – even though that will be very bitter ultimately. We find its analogue in worldly matters as well. A fool, and especially one whose weak intellect renders him ill, loves to grab and eat that which is pleasant to his palate, forgetting that this will often make him sick. On this our Rabbis, of blessed memory, said: “Who is wise? One who sees what will develop” (Avot 4:1). This is also the meaning of the saying of our Rabbis, of blessed memory (Sota 3a): “A person does not sin unless a spirit of foolishness enters into him.” This is a person’s primary labor in His service, may He be blessed: to think and consider the reverence of God and the fear of His punishment by means of the Mussar books and the aggadot of our Sages, of blessed memory, to the point that he hears it with his own ears and virtually sees it with his own eyes – the quantitatively and qualitatively great punishment standing before him. As our Rabbis, of blessed memory, say (Sanhedrin 7a): “At all times, a judge should see (given his likelihood to err; the same applies to every person vis-à-vis safeguarding against serious harm from grave sins) as if…hell is open below him.” If a person acts this way and his heart truly understands, he will repent and be healed. Indeed, man’s wickedness on the earth is great. No one seeks righteousness, and no one is truly enlightened through the fear of God, setting aside fixed times to work on the fear of God, drawing out waters of understanding from the concealed faith that is hiding in the inner recesses of the heart, to broaden it and sustain it and to give it force and strength, to place on its shoulders the authority to govern the external limbs that they not breach its fences but act in accordance with the Torah. This, then, is the second type of transgression. It has no analogue in worldly matters; there is no person who, when faced with burdening troubles, would not take time to think about how to escape them. Moreover, it is not caused by lust. This sin is dumbfounding (pelia), and its name is unknowable (peli), for [in committing the sin] the sinner does not pay attention to and consider his punishment through fear of God and Mussar. Such sins cannot be of the first type, for desire is not strong enough to be their cause. Rather, they stem from the spiritual impurity that coils around a person to cause him to sin. And especially in sins of speech, “for what benefit is there to the one who speaks evil?,” as stated by our Rabbis, of blessed memory (Arakhin 15b). See further there.