T. Austin-Sparks explores the profound concept of 'mystery' as presented in Ephesians, emphasizing that while God has revealed His will and the mystery of Christ, true understanding requires divine illumination. He distinguishes between the mystery of Christ and the mystery of iniquity, highlighting that both involve a supernatural presence in human form. Sparks asserts that spiritual growth comes from recognizing the corporate nature of the Church as the Body of Christ, which is essential for believers to experience fullness in their faith. He encourages the congregation to seek revelation from God to truly grasp their identity and purpose within the Church. Ultimately, the sermon calls for a deeper understanding of the Church's heavenly calling and the necessity of unity among believers for spiritual enlargement.