Nigel Lee emphasizes the importance of spiritual integrity and discernment in 'The Sermon on the Mount (4)', urging believers to examine their own lives before judging others. He highlights the necessity of seeking God's kingdom first and warns against the dangers of a critical spirit, encouraging self-reflection and personal accountability. Lee also discusses the significance of building one's life on the solid foundation of God's Word, contrasting it with the futility of a life built on sand. The sermon concludes with a call to action, reminding listeners that true faith is demonstrated through obedience and the fruits of one's life. Lee's message challenges the congregation to make conscious choices in their spiritual journey.