John Gill preaches on the theme of God's elect as chosen vessels of salvation, using the story of a widow and her miraculous supply of oil to illustrate how God's grace fills empty vessels. He emphasizes that while all humans are initially empty of grace, the elect are chosen to receive God's mercy and grace, which transforms them into vessels fit for His use. Gill explains that the oil symbolizes God's grace, which is abundant and everlasting, and that the filling of these vessels will continue until the last chosen vessel is filled. He encourages believers to trust in God's provision and to care for those in need, particularly the families of prophets.