J.R. Miller emphasizes the vital role of a Sunday School teacher's spiritual life in effectively guiding children in their faith. He highlights that teaching is not merely about methods or knowledge, but about embodying Christ's love and character, as teachers represent Him to their students. Miller stresses the importance of preparation, both spiritually and academically, and the necessity of a genuine love for Christ and for the children being taught. He warns that the responsibility of shaping young lives is profound, and teachers must be diligent in their own spiritual growth to positively influence their students.