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.