Gnosticism is a modern category used for defining a set of second-and third-century C.E. schools of thought and trends that have in common gnosis, a peculiar form of revealed knowledge that leads to salvation, having in itself both its value and its basis. In opposition to faith, gnosis takes root in the experience, generally human, of perceiving a division, a split between the self and the world, between the self and God, and between the self as a founding reality and the empirical ego. As global and absolute knowledge, gnosis aims at overcoming these dichotomies, recovering the individual's threatened integrity and restoring the lost unity of being.
Gnostic forms of knowledge leading to salvation are present in several religious traditions, theistic or not, such as Hinduism, Buddhism, Hebrew kabbalah, or Islamic esoteric traditions. However, in the Gnosticism of the second century C.E. a complex historical reality occurred, characterized by specific identity features, demonstrated by the fact that the holders of gnosis called themselves gnostikoi.
The plurality of available sources, from the Nag Hammadi texts to the writings of those fathers of the church, the so-called heresiologists who fought against Gnosticism as a heresy, makes difficult a reconstruction both of its origins and its history. The use of the category "Gnosticism" has been criticized because it provides an overview that hides the complexity of ancient historical reality by imposing an alleged unity to phenomena that were very different. However, this category is a legitimate interpretative historical tool, the only one that grasps the distinctive and unifying feature of schools of thought and movements otherwise different and at times controversial.