T. Austin-Sparks emphasizes the concept of the 'hidden secret' in God's plan, contrasting it with the outward rituals and traditions of the Old Testament. He clarifies that the term 'secret' is used to denote something that was kept hidden, rather than a mystical or mysterious concept. Sparks points out that while the Old Testament contained shadows of the truth, the fullness of God's revelation through Christ has now been disclosed, shifting from an era of external observance to one of internal faith. He stresses that the fundamental principle of faith remains unchanged across both eras, highlighting that true understanding and relationship with God transcends mere ritual. The sermon ultimately calls for a deeper spiritual comprehension of God's unchanging purpose throughout history.