Mysticism allows the building of the Oracle, which doubles the effects of temples. The advance increases the number of citizens made content by temples from 1 to 2 (4 if combined with the Oracle).
It is a prerequisite of astronomy and philosophy.
Civilopedia entry[]
As populations and knowledge grew, a new class of spiritual advisors arose to satisfy the need for less formal, state-centered rites of worship. The priests and priestesses of Mysticism, often called oracles, claimed union with the divine through meditation and trance-like contemplation. Mysticism offered for the first time the hope of personal salvation and immortality, concepts important in the development of later Religions.