Watchman Nee emphasizes the necessity of walking in the Spirit as a vital aspect of the Christian experience, contrasting the states of being 'in Adam' and 'in Christ' with living 'in the flesh' versus 'in the Spirit'. He explains that while being in Christ is an objective truth, the subjective experience of walking in the Spirit is essential for manifesting the life of Christ within us. Nee illustrates that true Christian living is not about striving in our own strength but trusting the Holy Spirit to produce the life of Christ in us, leading to victory over sin and a fulfilling relationship with God. He encourages believers to recognize that Christ is our life and that the Holy Spirit empowers us to live out that reality daily. Ultimately, the sermon calls for a deeper reliance on the Holy Spirit to fulfill God's will in our lives.