John Gill expounds on 2 Peter 2:21, emphasizing that it would have been better for those who have known the way of righteousness to remain in ignorance than to turn away from the truth they once embraced. He explains that the knowledge of the Gospel, which leads to justification through faith in Christ, carries a greater responsibility and guilt when rejected. Gill highlights the danger of apostasy, where individuals who once followed the holy commandment of the Gospel return to their former sinful ways, thus aggravating their condition. The sermon warns against the serious consequences of turning away from the truth after having known it.