Genesis 4:7

If thou doest well - That which is right in the sight of God, shalt thou not be accepted? Does God reject any man who serves him in simplicity and godly sincerity? But if thou doest not well, can wrath and indignation against thy righteous brother save thee from the displeasure under which thou art fallen? On the contrary, have recourse to thy Maker for mercy; לפתח חטאת רבץ lappethach chattath robets, a sin-offering lieth at thy door; an animal proper to be offered as an atonement for sin is now couching at the door of thy fold.

The words חטאת chattath, and חטאת chattaah, frequently signify sin; but I have observed more than a hundred places in the Old Testament where they are used for sin-offering, and translated ἁμαρτια by the Septuagint, which is the term the apostle uses, 2 Corinthians 5:21 : He hath made him to be sin (ἁμαρτιαν, A Sin-Offering) for us, who knew no sin. Cain's fault now was his not bringing a sin-offering when his brother brought one, and his neglect and contempt caused his other offering to be rejected. However, God now graciously informs him that, though he had miscarried, his case was not yet desperate, as the means of faith, from the promise, etc., were in his power, and a victim proper for a sin-offering was lying (רבץ robets, a word used to express the lying down of a quadruped) at the door of his fold. How many sinners perish, not because there is not a Savior able and willing to save them, but because they will not use that which is within their power! Of such how true is that word of our Lord, Ye will not come unto me that ye might have life!

Unto thee shall be his desire, etc. - That is, Thou shalt ever have the right of primogeniture, and in all things shall thy brother be subject unto thee. These words are not spoken of sin, as many have understood them, but of Abel's submission to Cain as his superior, and the words are spoken to remove Cain's envy.

4:7 If thou dost well, shalt thou not be accepted? — Either, 1. If thou hadst done well, as thy brother did, thou shouldest have been accepted as he was. God is no respecter of persons; so that if we come short of acceptance with him, the fault is wholly our own. This will justify God in the destruction of sinners, and will aggravate their ruin. There is not a damned sinner in hell, but if he had done well, as he might have done, had been a glorified saint in heaven. Every mouth will shortly be stopt with this. Or, 2. If now thou do well: if thou repent of thy sin, reform thy heart and life, and bring thy sacrifice in a better manner; thou shalt yet be accepted. See how early the gospel was preached, and the benefit of it here offered even to one of the chief of sinners. He sets before him death and a curse; but, if not well - Seeing thou didst not do well, not offer in faith, and in a right manner, sin lieth at the door - That is, sin only hinders thy acceptance. All this considered, Cain had no reason to he angry with his brother, but at himself only.

Unto thee shall be his desire — He shall continue in respect to thee as an elder brother, and thou, as the first-born, shall rule over him as much as ever. God's acceptance of Abel's offering did not transfer the birth-right to him, (which Cain was jealous of) nor put upon him that dignity, and power, which is said to belong to it, Genesis 49:3.