T. Austin-Sparks reveals the profound mystery of the 'One New Man' as articulated in Ephesians, emphasizing its significance in the context of biblical history and the nature of Christ. He explains how this concept encapsulates the essence of true Christianity, highlighting the transition from the old creation to the new creation in Christ, where distinctions are erased and unity is found in Him. Sparks draws connections between various scriptures, illustrating how the mystery of manhood according to Christ has been a governing theme throughout the Bible, culminating in the Church as the Body of Christ. He emphasizes the work of the Holy Spirit in forming this new man, which involves evangelism, spiritual growth, and the ultimate expression of Christ in the Church. The sermon culminates in the understanding that this revelation was hidden until now, showcasing God's eternal purpose for humanity.