From Wikipedia:
The work and writings of Jung from the 1940s onwards focused on alchemy.
In 1944 Jung published Psychology and Alchemy, where he analyzed the alchemical symbols and showed a direct relationship to the psychoanalytical process. He argued that the alchemical process was the transformation of the impure soul (lead) to perfected soul (gold), and a metaphor for the individuation process.
In 1963 Mysterium Coniunctionis first appeared in English as part of in The Collected Works of C. G. Jung. Mysterium Coniunctionis was Jung's last book and focused on the "Mysterium Coniunctionis" archetype, known as the sacred marriage between sun and moon. Jung argued that the stages of the alchemists, the blackening, the whitening, the reddening and the yellowing, could be taken as symbolic of individuation — his favourite term for personal growth.
Jung saw the human psyche as "by nature religious" and made this religiousness the focus of his explorations. Jung is one of the best known contemporary contributors to dream analysis and symbolization. His influence on popular psychology, the "psychologization of religion", spirituality and the New Age movement has been immense. - Emphasis added