John Gill emphasizes the significance of Christ as the chief cornerstone, as prophesied in Isaiah 28:16, highlighting His divine selection and preciousness to both God and humanity. He explains that Christ not only strengthens and adorns the spiritual building of the church but also unites believers across all ages and places. Gill reassures that those who believe in Christ will not be ashamed or confounded, as their faith provides safety and confidence both in this life and the next. The sermon underscores the importance of recognizing Christ's role in our spiritual lives and the assurance that comes from trusting in Him.