D.L. Moody emphasizes that joy is a powerful fruit of the Spirit and a privilege for every Christian. He explains that true joy comes from believing in the glad tidings of the Gospel, as demonstrated by the joy in Samaria when Philip preached. Moody highlights that joy is rooted in our relationship with Jesus, who provides assurance and grounds for our joy, particularly the knowledge that our names are written in heaven. He cautions against seeking joy apart from Christ, asserting that genuine joy can only be found in Him. Ultimately, Moody encourages believers to embrace the joy that comes from their faith and active service in God's work.