John Gill emphasizes the importance of brotherly love as a command from God, urging believers to love one another in the face of false teachings. He explains that love is a gift from God, a fruit of the Spirit, and a sign of true regeneration. Those who love demonstrate their knowledge of God and their relationship with Christ, as love for the brethren reflects the grace they have received. Gill encourages the faithful to stand united in truth and love, opposing errors with the strength of their shared faith.