Gnosticism (derived from Greek gnōsis meaning "knowledge") refers to a philosophical and religious movement in the Greco-Roman world that claimed that the path to salvation is through secret knowledge. Gnostic thought, having both Jewish and ancient Iranian religious origins, eventually came to full form within the context of Christianity in the 2nd century C.E. In the 2nd and 3rd centuries "gnosticism" was often the generic label for Christian heresy. Many of the early Christian councils and creeds were shaped in the effort to clarify Christian doctrine in the face of the gnostic challenge. Even though there is great diversity within gnostic theology, doctrine, and rituals, one common element is the claim that redemption occurs through access to esoteric knowledge that can only be acquired by divine revelation and passed on to initiates. Gnosticism is often organized into schools of thought that pass on the esoteric knowledge from one generation to the next. The authoritative teaching of these schools, including its interpretation and transmission, is often kept secret.
Scott Billings explains what it means to be a Gnostic in today's world.
Unknown God, emanations, demiurges
Various, including Apocryphon of John, Epistle of Eugnostos, Nag Hammadi texts, and Corpus Hermeticum