This is the question that arises in my mind as I read Genesis 4:1-16. Both Cain and Abel made an offering to Yahweh, but Cain wasn't accepted. Here's the text of Genesis 4:1-5:
והאדם ידע את חוה אשׁתו ותהר ותלד את קין ותאמר קניתי אישׁ את יהוה ותסף ללדת את אחיו את הבל ויהי הבל רעה צאן וקין היה עבד אדמה ויהי מקץ ימים ויבא קין מפרי האדמה מנחה ליהוה והבל הביא גם־הוא מבכרות צאנו ומחלבהן וישׁע יהוה אל הבל ואל מנחתו ואל קין ואל מנחתו לא שׁעה ויחר לקין מאד ויפלו פניו
Now Adam knew Eve his wife, and she conceived and bore Cain, saying, “I have gotten a man with the help of the Lord.” And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a worker of the ground. In the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard. So Cain was very angry, and his face fell.
I've always assumed that the problem must have been with Cain's offering. This interpretation goes back at least as far as the Septuagint (οὐκ, ἐὰν ὀρθῶς προσενέγκῃς, ὀρθῶς δὲ μὴ διέλῃς, ἥμαρτες; Gen 4:7). It's an understandable reading. That is, if God rebukes Cain, then it must have been for something he has already done. This is how Nahum Sarna explains it in his JPS Torah Commentary.
The reason for God's different reactions may be inferred from the descriptions of the offerings: Abel's is characterized as being "the choicest of the firstlings of his flock"; Cain's is simply termed as coming "from the fruit of the soil," without further details. Abel appears to have demonstrated a quality of heart and mind that Cain did not possess. Cain's purpose was noble, but his act was not ungrudging and openhearted. (Nahum M. Sarna, Genesis, JPS Torah Commentary, 32.)
As I read over the text yesterday, I became convinced that there is a better explanation. The story itself doesn't beckon us to look back at Cain's sacrifice and speculate about his attitude in offering his gift.
We aren't told explicitly what the issue was, but what we do know is the rest of the story. In the very next scene, Cain decided to have a chat with his brother. They ended up out in a field by themselves, and Cain killed Abel. We assume the anger he felt toward his brother was motivated by "God's favoritism." But could it not be the other way around?
After reading verse 5, I think we are supposed to feel like we have missed something, but perhaps we are right where the narrative intends us to be. We need to keep reading to find out what went wrong at the altar.
Here is 4:6-8, the rest of the story:
ויאמר יהוה אל קין למה חרה לך ולמה נפלו פניך הלוא אם תיטיב שׂאת ואם לא תיטיב לפתח חטאת רבץ ואליך תשׁוקתו ואתה תמשׁל בו ויאמר קין אל הבל אחיו ויהי בהיותם בשׂדה ויקם קין אל הבל אחיו ויהרגהו
The Lord said to Cain, “Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it.” Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him.
Yahweh's question in verse 7 points us forward. "If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it." I agree with Sarna. The issue is the state of Cain's heart, but as Yahweh speaks with Cain, it sounds more like a confrontation at a the crossroads, rather than a rebuke.
God knew Cain's heart was not right toward his brother. This was the issue at the altar, but more is at stake. Cain has not yet allowed his anger to turn to action. Sin is crouching at the door, and Cain must decide if he will let it have its way with him. The very next words reveal what Cain chose to do.
Cain wasn't accepted because murder was already alive and well inside him. The sin for which he is renown was already conceived in his heart, and Yahweh therefore wouldn't accept his worship.