John Gill emphasizes the critical importance of acknowledging the true nature of Christ, including His deity, humanity, and roles as Savior and Messiah. He warns that any spirit or doctrine that denies these truths is not from God and is aligned with the spirit of antichrist. Gill highlights that such false teachings are already present in the world, serving as forerunners to the ultimate antichrist. The sermon calls believers to remain steadfast in their faith and understanding of who Jesus is to avoid being led astray by deceptive doctrines.