John Nelson Darby emphasizes the importance of recognizing the distinction between the means of communicating Christ's religion and the rule of faith, arguing that true saving faith comes solely from the direct authority of God's word. He critiques Romanism for relying on church authority rather than the scriptures, asserting that faith must be rooted in God's testimony rather than human interpretation. Darby highlights the inadequacies of the Roman Catholic Church in preserving the authenticity of scripture and maintaining unity, ultimately concluding that only the Spirit of God can produce true unity among believers.