This posuk can be read as a dialogue between Hashem and Avrohom. After he is told to count the stars in the heaven, Avrohom proceeds to do the impossible and begins to count. Hashem then stops him and says, “do you really think you can count the stars in heaven? So will be your seed.”
Just as you attempted to fulfill my “impossible” command without question, so too will be your seed. They will observe My commandments without seeking a reason or rationale for their fulfillment.
“One’s will should not be measured by ability, but rather ability should be measured by one’s will.” Very often a mitzvah or good deed seems very hard to fulfill, either because of lack of time or strength or what ever excuse may arise. If one will make a proper self-evaluation he will see that he is not lacking in ability but rather in proper will and desire to perform the mitzvah or good deeds.
