J.R. Miller emphasizes the secret of gladness in the Christian life, asserting that true joy is not dependent on circumstances but is rooted in God's love and grace. He explains that while Christians will face trials and sorrows, their joy can remain unshaken, as it is derived from a deep relationship with God. Miller encourages believers to cultivate a spirit of gladness through gratitude, a happy home, and a good conscience, while also recognizing the beauty of the world around them. He concludes that the essence of Christian gladness is a learned response to life’s challenges, reflecting the light of Christ even in darkness.