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.
Plato The Statesman 258e— Stranger: In this way, then, divide all science into two arts, calling the one practical (praktikos), and the other purely intellectual (gnostikos). Younger Socrates: Let us assume that all science is one and that these are its two forms.
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".
Hellenistic Jewish literature
The Greek word gnosis (knowledge) is used as a standard translation of the Hebrew word "knowledge" (דעת da'ath) in the Septuagint, thus:
— Proverbs 2.6
Paul distinguishes "knowledge" (gnosis) and "knowledge falsely so-called" (pseudonymos gnosis).
In the writings of the Greek Fathers
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.
In Eastern Orthodox thought
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.
- Stanley E. Porter; David Yoon (2016). Paul and Gnosis. BRILL. p. 9. ISBN 978-90-04-31669-0.
- Kurt Rudolph (2001). Gnosis: The Nature and History of Gnosticism. A&C Black. p. 2. ISBN 978-0-567-08640-2.
- Gnosticism, Encyclopædia Britannica
- Liddell Scott entry γνῶσις, εως, ἡ, A. seeking to know, inquiry, investigation, esp. judicial, “τὰς τῶν δικαστηρίων γ.” D.18.224; “τὴν κατὰ τοῦ διαιτητοῦ γdeetr.” Id.21.92, cf. 7.9, Lycurg.141; “γ. περὶ τῆς δίκης” PHib.1.92.13 (iii B. C.). 2. result of investigation, decision, PPetr.3p.118 (iii B. C.). II. knowing, knowledge, Heraclit.56; opp. ἀγνωσίη, Hp. Vict.1.23 (dub.); opp. ἄγνοια, Pl.R.478c; “ἡ αἴσθησις γ. τις” Arist.GA731a33: pl., “Θεὸς γνώσεων κύριος” LXX 1 Ki.2.3. b. higher, esoteric knowledge, 1 Ep.Cor.8.7,10, Ep.Eph.3.19, etc.; “χαρισάμενος ἡμῖν νοῦν, λόγον, γνῶσιν” PMag.Par.2.290. 2. acquaintance with a person, πρός τινα Test. ap.Aeschin.1.50; “τῶν Σεβαστῶν” IPE1.47.6 (Olbia). 3. recognizing, Th.7.44. 4. means of knowing, [“αἱ αἰσθήσεις] κυριώταται τῶν καθ᾽ ἕκαστα γ.” Arist.Metaph.981b11. III. being known, γνῶσιν ἔχει τι, = γνωστόν ἐστι, Pl.Tht.206b. 2. fame, credit, Hdn.7.5.5, Luc.Herod.3. IV. means of knowing: hence, statement in writing, PLond.5.1708, etc. (vi A. D.). V. = γνῶμα, Hsch. s. h. v.
- Pagels, Elaine (1995). The Origin of Satan. Allen Lane, The Penguin Press. p. 167.
- LSJ entry γνωστ-ικός, ή, όν, A. of or for knowing, cognitive: ἡ -κή (sc. ἐπιστήμη), theoretical science (opp. πρακτική), Pl.Plt.258b.c., etc.; τὸ γ. ib.261b; “ἕξεις γ.” Arist.AP0.100a11 (Comp.); “γ. εἰκόνες” Hierocl.in CA25p.475M.: c. gen., able to discern, Ocell. 2.7. Adv. “-κῶς” Procl.Inst.39, Dam.Pr.79, Phlp.in Ph.241.22.
- In Perseus databank 10x Plato, Cratylus, Theaetetus, Sophist, Statesman 2x Plutarch, Compendium libri de animae procreatione + De animae procreatione in Timaeo, 2x Pseudo-Plutarch, De musica
- Cooper and Hutchinson. "Introduction to Politikos." Cooper, John M. & Hutchinson, D. S. (Eds.) (1997). Plato: Complete Works, Hackett Publishing Co., Inc. ISBN 0-87220-349-2.
- Plato. Plato in Twelve Volumes, Vol. 12 translated by Harold N. Fowler. Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1921.
- George Robert Stow Mead, and Stephen Ronan. The Complete Echoes from the Gnosis. London, Chthonios Books, 1987, p. 113.
- feminine nominative adjective
- New Testament studies: Society for New Testament Studies – 1981 "see also the more extensive analysis of gnosis in Philo by Hans Jonas, Gnosis und spatantiker Geist 11/1"
- Donald K. McKim, Westminster dictionary of theological terms, 1996, p. 39
- A concise dictionary of theology by Gerald O'Collins, Edward G. Farrugia p. 130 Publisher: T. & T. Clark Publishers (2004) ISBN 978-0-567-08354-8 
- "Spiritual knowledge is the state of spiritual theoria, when one sees invisibly and hears inaudibly and comprehends incomprehensibly the glory of God. Precisely then comprehension ceases and, what is more, he understands that he does not understand. Within the vision of the uncreated Light man also sees angels and Saints and, in general, he experiences communion with the angels and the Saints. He is then certain that resurrection exists. This is the spiritual knowledge which all the holy Prophets, the Apostles, Martyrs, ascetics and all the Saints of the Church had. The teachings of the Saints are an offspring of this spiritual knowledge. And, naturally, as we said earlier, spiritual knowledge is a fruit of the vision of God. "THE ILLNESS AND CURE OF THE SOUL" Metropolitan Hierotheos of Nafpaktos 
- St. Symeon the New Theologian in Practical & Theological Discourses, 1.1 The Philokalia Volume Four: When men search for God with their bodily eyes they find Him nowhere, for He is invisible. But for those who ponder in the Spirit He is present everywhere. He is in all, yet beyond all
- Faith And Science In Orthodox Gnosiology And Methodology by George Metallinos "The scientist and professor of the knowledge of the Uncreated, in the Orthodox Tradition, is the Geron/Starets (the Elder or Spiritual Father), the guide or "teacher of the desert." The recording of both types of knowledge presupposes empirical knowledge of the phenomenon. The same holds true in the field of science, where only the specialist understands the research of other scientists of the same field. The adoption of conclusions or findings of a scientific branch by non-specialists (i.e. those who are unable to experimentally examine the research of the specialists) is based on the trust of the specialists credibility. Otherwise, there would be no scientific progress. The same holds true for the science of faith. The empirical knowledge of the Saints, Prophets, Apostles, Fathers and Mothers of all ages is adopted and founded upon the same trust. The patristic tradition and the Church's Councils function on this provable experience. There is no Ecumenical Council without the presence of the glorified/deified (theoumenoi), those who see the divine (this is the problem of the councils of today!) Orthodox doctrine results from this relationship." University of Athens - Department of Theology
- The Philokalia Volume Four Palmer, G.E.H; Sherrard, Philip; Ware, Kallistos (Timothy). ISBN 0-571-19382-X, glossary, p. 434, Spiritual Knowledge (γνῶσις): the knowledge of the intellect (q.v.). As such, it is knowledge inspired by God, as insight (noesis) or revelational, intuitive knowledge (see gnosiology) and so linked with contemplation and immediate spiritual perception.
- Glossary of terms from the Philokalia p. 434 the knowledge of the intellect as distinct from that of the reason(q.v.). Knowledge inspired by God, and so linked with contemplation (q.v.) and immediate spiritual perception.
- The Mystical Theology of the Eastern Church, SVS Press, 1997. (ISBN 0-913836-31-1) James Clarke & Co Ltd, 2002. (ISBN 0-227-67919-9) p. 218
- Magris 2005, pp. 3515–3516.
- The Social World of the First Christians (1995) ISBN 0-06-064586-5, essay "Prolegomena to the Study of Ancient Gnosticism" by Bentley Layton 
- Magris, Aldo (2005), "Gnosticism: Gnosticism from its origins to the Middle Ages (further considerations)", in Jones, Lindsay (ed.), MacMillan Encyclopedia of Religion, MacMillan