In 1895, Marmion gave a retreat for a small group of nuns. The notes for those talks contained in kernel an idea he would develop during the next 20 years – meditating upon it in prayer, and refining and polishing it in the many talks he gave as a popular retreat master. In its finished form it became "Christ, the Life of the Soul" (1917) – a book that was first published privately, but then rapidly, unexpectedly, became an "overwhelming success" in the Catholic world.