Charles Finney emphasizes the necessity of spiritual illumination to truly understand the law of God, as expressed in Psalm 119:18. He explains that the term 'law' encompasses the entire revealed will of God, and the request to 'open mine eyes' signifies a plea for spiritual light to perceive the wonders within Scripture. Finney outlines the implications of this request, including the need for divine assistance to grasp spiritual truths and the transformative consequences of having one's eyes opened, which leads to a deeper understanding of oneself and God. He concludes that such illumination not only enriches personal faith but also sets believers apart from those who remain spiritually blind.