John Gill preaches on the fullness of Christ as the Mediator, emphasizing that all fullness dwells in Him as the embodiment of divine attributes and grace. He explains that this fullness includes the personal fullness of deity, the relative fullness of the church, and the fullness of grace and glory that Christ distributes to believers. Gill highlights that this fullness is a result of the Father's good pleasure, ensuring that Christ is the source of all spiritual blessings and the means through which believers receive grace. Ultimately, he encourages the congregation to seek their needs in Christ, who is the head of all things and the mediator of the new covenant.