Religious monism has two forms: atheism and pantheism. Both deny that there is a transcendent deity. Pantheism posits a deity that is immanent to the world and on which the world completely depends. Atheism states that there is no deity at all. Critics of pantheism sometimes conflate it with atheism, on the grounds that a true God must be transcendent.
Among the most ancient forms of pantheism is Brahmanism. Many of its main tenets were expressed in the Upanishads and systematized by the Vedanta-sutras. Its beginning is traced to the seventh-century thinker Gaudapada, who denied individuation and plurality. Appearances, as well as individual minds, are only temporary manifestations of the all-soul. Master Sankara (c. 700–750, India), author of commentaries on the Brahma-sutra, on parts of the Upanishads, and on the Mandukya-karika, studied with a pupil of Gaudapada. For Sankara, only the Brahman is real, and plurality and difference are an illusion.
Gnosticism is the name given to various doctrines of salvation through knowledge. The first gnostic sects were pre-Christian. Scholars argue about Persian Mazdeism, Greek mysteries, Egyptian doctrines, or Babylonian astrology and religions as possible roots of Gnostic thought. Gnosticism came into contact with Judaism and early Christianity, borrowing some names and concepts, though refusing the main tenets. Ancient Gnosticism held that everything flows from one purely spiritual principle. The origins of matter were explained as a flaw in a long line of successive emanations from the one principle. While matter is impure, its existence is temporary: Gnostic eschatology states that ultimately the original unity and purity will be restored. Gnostic sects include the Syrian school and the Alexandrian school (early second century).
Only rarely do Judeo-Christian forms of mysticism accept monism. John Scotus Erigena (c. 810–c. 877) in Ireland, Johannes Eckehart (Meister Eckehart; c. 1260–?1327) in Germany, and Nicholas of Cusa (1401–1464) endorsed forms of pantheism. The same is true for Islamic Sufism, which appeared as a reaction to the overly worldly tendencies of Islam in the late seventh century C.E. While most Sufi authors stressed personal discipline, asceticism, and purity as necessary elements to prepare the soul to know and unite with a transcendent God (which hardly qualify them as monist), it is interesting to mention figures that stressed a metaphysical unity of all beings in God. In the thirteenth century, the Spanish-born Ibn al-'Arabi (1165–1240) created a theory of the "Unity of Being." According to this theory, all existence is one, a manifestation of the underlying divine reality. Sufi currents that stressed the unity of all reality were present also in Indian Muslim communities in the late sixteenth and early seventeenth centuries. Mainstream Sufi authors reacted against monistic trends, stressing that a unity of vision (the mystical experience) did not correspond to a unity of reality.