John Gill emphasizes the importance of brotherly love by contrasting it with the hatred exemplified by Cain, who, influenced by evil, murdered his brother Abel out of envy. Gill explains that Cain's actions stemmed from his own wickedness and the acceptance of Abel's righteous sacrifice by God, which incited Cain's jealousy. He warns against allowing such hatred to take root in our hearts, as it leads to destructive actions and a separation from God. The sermon highlights the need for self-examination of our motives and the importance of faith in our works. Ultimately, Gill calls believers to embody love and righteousness, avoiding the path of Cain.

Ver. 12. Not as Cain,.... That is, let us not be like him, or do as he did, hate the brethren. The apostle illustrates brotherly love by its contrary, in the instance of Cain, who was the first instance and example of hatred of the brethren, and of fratricide, and a very detestable one, by which he would dissuade from so vile and abominable a practice: [who] was of that wicked one; Satan, a child of his, an imitator of him, one that appeared to be under his influence, and to belong unto him. So the Jews say of Cain {n}, that "he was of the side of the serpent (the old serpent the devil); and as the way of the serpent is to slay and to kill, so Cain immediately became a murderer.'' And again, "because Cain came from the side of the angel of death, he slew his brother {o};'' though they say that he afterwards repented, and became worthy of paradise {p}. And slew his brother; see Ge 4:8. According to the tradition of the Jews {q} he struck a stone into his forehead, and killed him: and wherefore slew he him? what was the cause and occasion of it? what moved him to it? because his own works were evil, and his brother's righteous; or "his work", as the Ethiopic version reads: the sacrifice which he offered up, which, though it was not evil as to the matter and substance of it, yet was so, being offered with an evil mind, and with an hypocritical heart, and without faith in the sacrifice of Christ, and so was unacceptable to God; whereas, on the other hand, the sacrifice his brother brought was offered up in the faith of Christ, by which he obtained a testimony that he was righteous, and that the work he did was a righteous work, being done in faith, and so was acceptable to God; which Cain perceiving, was filled with envy, and this put him upon killing him. The Jews {r} relate the occasion of it after this manner; "Cain said to Abel his brother, come, and let us go out into the open field; and when they were both out in the open field, Cain answered and said to Abel his brother, there is no judgment, nor Judge, nor another world; neither will a good reward be given to the righteous, nor vengeance be taken on the wicked; neither was the world created in mercy, nor is it governed in mercy; or why is thy offering kindly accepted, and mine is not kindly accepted? Abel answered and said to Cain, there is judgment, and there is a Judge, and there is another world; and there are gifts of a good reward to the righteous, and vengeance will be taken on the wicked; and the world was created in mercy, and in mercy it is governed, for according to the fruit of good works it is governed; because that my works are better than thine, my offering is kindly accepted, and thine is not kindly accepted; and they both strove together in the field, and Cain rose up against Abel his brother, and slew him.'' In the Hebrew text in Ge 4:8; there is an extraordinary large pause, as if a discourse of this kind, which passeth between the two brothers, was to be inserted. Philo the Jew says {s}, that in the contention or dispute between Cain and Abel, Abel attributed all things to God, and Cain ascribed everything to himself; so that the controversy was about grace and works, as now; and as then Cain hated his brother upon this account, so now carnal men hate and persecute the saints, because they will not allow their works to be the cause of justification and salvation: and from hence also it may be observed, that a work may be, as to the matter of it, good, and yet as to its circumstances, and the end and view of it, evil. {n} Midrash Ruth in Zohar in Gen. fol. 42. 4. {o} Zohar in ib. fol. 43. 1. {p} Ib. fol. 41. 1, 2. {q} Targum Jon. in Gen. iv. 8. Pirke Eliezer, c. 21. {r} Targum Hieros. & Jon. in Gen. iv. 8. {s} Quod Det. Potior. p. 161.