Serving Hashem Through Truth
ברש"י מצות קלות שאדם דש כו' ובמד' תנחומא בשמרם עקב רב מה רב טובך כו' והטעם כי מצות קלות אין האדם עושה עבור השכר רק לעשות רצון הש"י ולזאת אף כי וודאי הכל במדה ויש שכר יותר על מצוה חמורה
Rashi explains that the word 'eikev' ('heel') alludes to the lighter mitzvos, which a person tends to trample under his heel, and the Midrash Tanchuma on the verse 'in observing them there is great reward (eikev rav)' connects this to the verse 'How great is Your goodness which You have stored away for those who fear You' (Tehillim 31:20). The reason is that with the lighter mitzvos a person does not perform them for the sake of reward, but only to fulfill the will of Hashem; and therefore, even though everything is certainly measured precisely and there is greater reward for a weightier mitzvah,
Rashi reads the word 'eikev' (literally 'heel') as a hint to the lighter mitzvos that people trample underfoot, and the Midrash ties this to the 'great goodness stored away.' The Sfas Emes explains that since these lighter mitzvos are done purely to do Hashem's will rather than to earn a payoff, they reveal a special quality - even though, by strict measure, weightier mitzvos earn more reward.
מ"מ הקב"ה צופן שכר טוב ליראיו היינו המקיימים מצותיו עבור יראתו לא עבור השכר
nonetheless the Holy One, Blessed is He, stores away a great reward 'for those who fear Him' (Tehillim 31:20) - meaning those who fulfill His mitzvos out of yiras Shamayim, and not for the sake of the reward.
Hashem reserves a hidden, surpassing reward specifically for those who serve Him out of yiras Shamayim rather than for what they will get out of it.
וז"ש ושמר כו' לך ישמור לך הבטחתו
This is the meaning of 'and He will guard (v'shamar)' (Devarim 7:12) - Hashem will guard for you His promise.
The verse's promise that Hashem 'will guard' refers to Hashem faithfully keeping for Bnei Yisrael the promise He made to them.
והכלל כי לכל ישראל יש חלק בעוה"ב ובאמת בנ"י מיוחדים הם להקב"ה
The general principle is that every member of Bnei Yisrael has a portion in Olam Haba, and in truth Bnei Yisrael are uniquely bound to the Holy One, Blessed is He.
Fundamentally, every Jew already has a share in Olam Haba, because Bnei Yisrael are intrinsically and uniquely connected to the Holy One, Blessed is He.
ורק שצריך האדם להיות אצל האמת
It is only that a person must position himself alongside the emes,
The one thing required of a person is to plant himself next to the emes - to stand on the side of truth.
וכפי מה שמקרב עצמו לדרך האמת ממילא זוכה לרב טוב:
and according to how much he draws himself near to the path of emes, he automatically merits great goodness.
The closer a person draws himself to the path of emes, the more he automatically becomes worthy of the great goodness that Hashem has stored away.
Summary: The Sfas Emes opens with Rashi's reading of 'eikev' as a reference to the lighter mitzvos that people trample under their heel, which the Midrash links to the 'great goodness' Hashem stores away for those who fear Him. He explains that these lighter mitzvos are precious precisely because a person performs them not for reward but solely to do the will of Hashem; and so, even though by strict measure a weightier mitzvah earns more, Hashem reserves a hidden, surpassing reward for those who serve Him out of yiras Shamayim alone. This is the promise that Hashem 'will guard' - He faithfully keeps His word to Bnei Yisrael, who are uniquely bound to Him and each have a portion in Olam Haba. All that is asked is that a person position himself alongside the emes. To the degree that he draws himself near to the path of emes, he automatically merits that great goodness.