John Gill emphasizes the promise of eternal life as a fundamental aspect of the Gospel, highlighting that this promise originates from God the Father and is affirmed by Jesus Christ. He explains that this promise is secure and rests in the covenant of grace made before the world began, and it is essential for believers to hold firmly to this doctrine. The sermon underscores the importance of knowing and enjoying a relationship with both the Father and the Son, as this knowledge is integral to experiencing eternal life.