Gnosis is the common Greek noun for knowledge (γνῶσις, gnōsis, f.). The term is used in various Hellenistic religions and philosophies. It is best known from Gnosticism, where it signifies a knowledge or insight into humanity’s real nature as divine, leading to the deliverance of the divine spark within humanity from the constraints of earthly existence.
Gnosis is a feminine Greek noun which means "knowledge" or "awareness."  It is often used for personal knowledge compared with intellectual knowledge (εἶδειν eídein), as with the French connaître compared with savoir, the Spanish conocer compared with saber, the Italian conoscere compared with sapere, the German kennen rather than wissen, or the Modern Greek γνωρίζω compared with ξέρω.
A related term is the adjective gnostikos, "cognitive", a reasonably common adjective in Classical Greek. Plato uses the plural adjective γνωστικοί – gnostikoi and the singular feminine adjective γνωστικὴ ἐπιστήμη – gnostike episteme in his Politikos where Gnostike episteme was also used to indicate one's aptitude. The terms do not appear to indicate any mystic, esoteric or hidden meaning in the works of Plato, but instead expressed a sort of higher intelligence and ability analogous to talent.
In the Hellenistic era the term became associated with the mystery cults.
Gnosis is used throughout Greek philosophy as a technical term for experiential knowledge (see gnosiology) in contrast to theoretical knowledge or epistemology. The term is also related to the study of knowledge retention or memory (see also cognition), in relation to ontic or ontological, which is how something actually is rather than how something is captured (abstraction) and stored (memory) in the mind.
Irenaeus used the phrase "knowledge falsely so-called" (pseudonymos gnosis, from 1 Timothy 6:20) for the title of his book On the Detection and Overthrow of False Knowledge, that contains the adjective gnostikos, which is the source for the 17th-century English term "Gnosticism".
The Greek word gnosis (knowledge) is used as a standard translation of the Hebrew word "knowledge" (דעת da'ath) in the Septuagint, thus:
Paul distinguishes "knowledge" (gnosis) and "knowledge falsely so-called" (pseudonymos gnosis).
The fathers of early Christianity used the word gnosis (knowledge) to mean spiritual knowledge or specific knowledge of the divine. This positive usage was to contrast it with how gnostic sectarians used the word. This positive use carried over from Hellenic philosophy into Greek Orthodoxy as a critical characteristic of ascetic practices, through St. Clement of Alexandria, Irenaeus, Hippolytus of Rome, Hegesippus, and Origen.
Cardiognosis ("knowledge of the heart") from Eastern Christianity related to the tradition of the starets and in Roman Catholic theology is the view that only God knows the condition of one's relationship with God.
Gnosis in Orthodox Christian (primarily Eastern Orthodox) thought is the spiritual knowledge of a saint (one who has obtained theosis) or mystically enlightened human being. Within the cultures of the term's provenance (Byzantine and Hellenic) Gnosis was a knowledge or insight into the infinite, divine and uncreated in all and above all, rather than knowledge strictly into the finite, natural or material world. Gnosis is transcendental as well as mature understanding. It indicates direct spiritual, experiential knowledge and intuitive knowledge, mystic rather than that from rational or reasoned thinking. Gnosis itself is gained through understanding at which one can arrive via inner experience or contemplation such as an internal epiphany of intuition and external epiphany such as the Theophany.
In the Philokalia, it is emphasized that such knowledge is not secret knowledge but rather a maturing, transcendent form of knowledge derived from contemplation (theoria resulting from practice of hesychasm), since knowledge cannot truly be derived from knowledge, but rather, knowledge can only be derived from theoria (to witness, see (vision) or experience). Knowledge, thus plays an important role in relation to theosis (deification/personal relationship with God) and theoria (revelation of the divine, vision of God). Gnosis, as the proper use of the spiritual or noetic faculty plays an important role in Orthodox Christian theology. Its importance in the economy of salvation is discussed periodically in the Philokalia where as direct, personal knowledge of God (noesis; see also Noema) it is distinguished from ordinary epistemological knowledge (episteme—i.e., speculative philosophy).
Gnosticism originated in the late first century CE in nonrabbinical Jewish sects and early Christian sects. In the formation of Christianity, various sectarian groups, labeled "gnostics" by their opponents, emphasised spiritual knowledge (gnosis) of the Divine spark within, over faith (pistis) in the teachings and traditions of the various communities of Christians. Gnosticism presents a distinction between the highest, unknowable God, and the demiurge “creator” of the material universe. The Gnostics considered the most essential part of the process of salvation to be this personal knowledge, in contrast to faith as an outlook in their world view along with faith in the ecclesiastical authority. They were regarded as heretics by the Fathers of the early church.