motivation to pass the test

 On the way to the akeidah, Avraham took some things along:

 וַיִּקַּ֣ח בְּיָד֔וֹ אֶת־הָאֵ֖שׁ וְאֶת־הַֽמַּאֲכֶ֑לֶת

Rashi comments:

מאכלת – סכין, על שם שאוכלת בשר, כמה דאתה אומר: וחרבי תאכל בשר (דברים ל״ב:מ״ב), ושמכשרת בשר לאכילה.

דבר אחר: זו נקראת מאכלתא על שם שישראל אוכלין מתן שכרה.

It's called a מַּאֲכֶ֑לֶת and not a 'sakin' because we eat the rewards of this knife forever after.

What are Chazal trying to teach us with this derash?  That we enjoy eternal reward for akeidas Yitzchak?  Don't we know that without a derash on the single word מַּאֲכֶ֑לֶת? 

What Chazal were commenting on, says Sefas Emes, is how Avraham Avinu motivated himself for the test of the akeidah.  How does a person rise to such a challenge?

Chazal answer that וַיִּקַּ֣ח בְּיָד֔וֹ, the motivation that Avraham took in hand, is the מַּאֲכֶ֑לֶת, the fact that שישראל אוכלין מתן שכרה.  Avraham took to heart that his actions would reflect not just the strength of his own beliefs, but would reverberate through history and have an impact on future generations.  The relationship between present and future cuts both ways -- future generations draw inspiration and meaning from Avraham's passing the test of the akeidah, and Avraham in turn drew inspiration from the thought of future generations riding on his coattails.

The Sefas Emes uses this same idea elsewhere.  For example, the Midrash writes that had Reuvain known the Torah would write that he saved Yosef from the pit he would have carried him home to Yaakov on his shoulder.  It's not that Reuvain was out for fame and would have behaved differently had he known his actions would be front page news.  Sefas Emes explains that what Chazal are telling us is that Reuvain would have behaved differently had he known that his actions constitute "Torah," i.e. something that generations would be impacted by and learn from.  When you know that your actions have an impact on generations to come, it changes your behavior, it pushes you to do more.