4:4 וְהֶבֶל הֵבִיא גַם־הוּא מִבְּכֹרוֹת צֹאנוֹ וּמֵחֶלְבֵהֶן — and Abel for his part brought of the firstlings of his flock, their fat portions—In contrast to the Scripture’s silence with regard to Cain’s sacrifice, considerable detail is lavished on the quality of Abel’s offering. First of all, he offered his “firstlings,” which the Torah would later view comes from the best of one’s flock. Second, he offered the animals’ fattest parts, i.e., he sacrificed his choicest animals.
Note that Abel does not just offer the firstborn of his flock; he also offers even the very best of his flock—even if the animals weren’t necessarily the firstborn. Third, the verse intimates a clever pun in the words: וְהֶבֶל הֵבִיא גַם־הוּא (wüheºbel hëbî´ gam-hû´) — “he also brought himself.” A literal translation of the text indicates that Abel realized that the true sacrifice reflects the inner person and the heart of the person offering it.
But why was Cain’s sacrifice rejected? The Torah does not explicitly say, but the absence of detail spent describing Cain’s sacrifice indicates that it must have been quite ordinary. Even if his sacrifice was no less exemplary than his brother’s, it is also possible that his attitude marred the quality of the sacrifice.