John Nelson Darby emphasizes the significance of understanding the application of Christ's blood in the context of salvation, arguing against the notion that the blood must be re-applied for cleansing. He asserts that believers are perfected forever through Christ's single sacrifice, which clears their consciences and allows them to approach God without fear of condemnation. Darby clarifies that while the Old Testament practices involved blood sprinkling, the New Testament reveals that Christ's offering is sufficient and does not require repetition. He highlights the importance of faith in the efficacy of Christ's sacrifice, which provides a perfect conscience before God. The sermon serves as a reminder that true holiness and liberty in Christ come from recognizing the completeness of His work on the cross.