D.L. Moody emphasizes that true joy transcends personal salvation and is found in the joy of leading others to Christ. He describes three types of joy, highlighting that the joy of seeing others saved is the highest honor and privilege. Moody contrasts happiness, which is dependent on circumstances, with a deeper, unshakeable joy that persists through trials and is rooted in obedience to God. He encourages believers to seek joy in the Word of God, which should inspire them to serve others and engage in Christian work. Ultimately, he asserts that genuine joy compels action and service in the vineyard of the Lord.