The sermon explores the spiritual principle that one's inner state is determined by what they allow into their senses, drawing primarily from Luke 11 where Jesus teaches that the eye is the lamp of the body. It contrasts the light of God's Word with the darkness of worldly influences, emphasizing the critical need to guard what is seen and heard. The message highlights several functions of Scripture, including its ability to save souls, transform believers progressively, provide guidance, cleanse sin, establish strength, bring joy, and secure divine blessing.