Bitachon Despite Honest Fear
אא"ז מו"ר ז"ל הגיד פי' הכ' כי תאמר כו' רבים הגוים כו'
My grandfather, my master and teacher, of blessed memory, explained the meaning of the verse, "When you will say in your heart, 'These nations are more numerous than I; how can I drive them out?'" (Devarim 7:17).
The Sfas Emes opens by citing his grandfather's reading of the verse in which Bnei Yisrael wonder how they can possibly conquer nations greater and more numerous than themselves.
אף שיאמרו כן יזכרו ביצ"מ
Even though they will say this, they should remember the going out of Mitzrayim (yetzias Mitzrayim).
The answer the Torah gives is to recall yetzias Mitzrayim, where Hashem overpowered a mighty nation on their behalf.
וזה לכל איש ישראל אף שיש לו כמה הסתרות צריך להתחזק עצמו לבטוח בהשי"ת ושלא לירא מהם
And this is a lesson for every man of Yisrael: even though he has many concealments, he must strengthen himself to place his trust in Hashem and not to fear them.
This is a directive for every Jew: no matter how many things hide Hashem's hand from him, he must reinforce his bitachon in Hashem and refuse to be afraid.
וזה הבטחה אף למי שיש לו מורך לב כו'
And this is an assurance even for one who has faintheartedness of the heart.
The Torah's promise is aimed specifically at the person whose heart grows weak and timid in the face of the challenge.
וי"ל עוד פשוט כי תאמר כפשוטו כי מקודם צריך האדם לידע כי מעצמו אין לו זכות וצריך לירא מהם
And one may further explain it simply: "When you will say" is meant as it plainly reads, for first a man must know that of his own self he has no merit, and he must fear them.
On a simpler level, the verse is taken at face value: a person must first recognize that on his own he possesses no merit and genuinely has cause to fear.
רק עכ"ז לא יירא מהם ע"י שיזכור מיצ"מ כנ"ל
Only that, nonetheless, he should not fear them, by means of remembering yetzias Mitzrayim, as stated above.
Yet despite that honest fear, he must not let it overcome him, because remembering yetzias Mitzrayim shows him that Hashem carries the victory.
איכה אוכל להורישם היינו מצד עצמו לא הי' בכחו לולי הקב"ה עוזרו כנ"ל:
"How can I drive them out?" (Devarim 7:17) — that is to say, from his own self it was not within his power, were it not that the Holy One, Blessed is He, helps him, as stated above.
The phrase "how can I drive them out" expresses the truth that by his own strength he could never succeed, and only Hashem's help makes it possible.
Summary: The Sfas Emes, citing his grandfather, learns from the verse "When you will say in your heart, these nations are more numerous than I" (Devarim 7:17) that when Bnei Yisrael feel overwhelmed by foes greater than themselves, the remedy is to remember yetzias Mitzrayim. From this he draws a lesson for every Jew: however many concealments cloud his perception of Hashem, he must strengthen his bitachon and not give in to fear. He then offers a plainer reading: a person should honestly acknowledge that on his own he has no merit and real reason to fear, yet precisely then he must refuse to fear by recalling the redemption from Mitzrayim. The closing words "how can I drive them out" capture the truth that by his own strength he is powerless, and his success comes solely from the Holy One, Blessed is He, helping him.