עקב אשר שמע אברהם בקלי וישמר משמרתי מצותי חקתי ותורתי
In his commentary to the above pasuk, Sforno makes an important point. He notes that Hashem promised Yitzchak Avinu to multiply his offspring, grant his descendants the Land and bless them – all because of His oath to Avraham Avinu. We see here (explains Sforno) that z’chus Avos, merit of others (his father, Avraham Avinu) is invoked when Hashem speaks to Yitzchak. Not so with Avraham (who did not have z’chus Avos) or Yaakov. This is because, before Yitzchak was inspired to call upon the Name of Hashem (after Gerar), when Avimelech came to him and said, “We saw that…
