Gnosis is a feminine Greek noun which means "knowledge" or "awareness." It is often used for personal knowledge compared with intellectual knowledge (εἴδειν eídein), as with the Frenchconnaître compared with savoir, the Spanishconocer compared with saber, the Italianconoscere compared with sapere, the Germankennen 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. 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.
The difference and meaning of epignosis (Greek: έπἱγνωσις) contrasted with gnosis is disputed. One proposed distinction is between the abstract or fragmented knowledge (gnosis) and a clearer or more precise knowledge (epignosis). Other interpretations have suggested that 2 Peter is referring to an "epignosis of Jesus Christ", what J.B. Lightfoot described as a "larger and more thorough knowledge". Conversion to Christianity is seen as evidence of the deeper knowledge protecting against false doctrine.
However, not all Gnostic movements regarded the creator of the material universe as inherently evil or malevolent. For instance, Valentinians believed that the Demiurge is merely an ignorant and incompetent creator, trying to fashion the world as good as he can, but lacking the proper power to maintain its goodness. All Gnostics were regarded as heretics by the proto-orthodoxEarly Church Fathers.
In Mandaeism, the concept of manda ("knowledge", "wisdom", "intellect") is roughly equivalent to the Gnostic concept of gnosis.Mandaeism ('having knowledge') is the only surviving Gnostic religion from antiquity.: 15 Mandaeans formally refer to themselves as Nasurai (Nasoraeans) meaning guardians or possessors of secret rites and knowledge. The Mandaeans emphasize salvation of the soul through secret knowledge (gnosis) of its divine origin. Mandaeism "provides knowledge of whence we have come and whither we are going.": 531
Hellenistic Jewish literature
The Greek word gnosis (knowledge) is used as a standard translation of the Hebrew word "knowledge" (דעתda'ath) in the Septuagint, thus:
The Lord gives wisdom [ħokhma] (sophia), from his face come knowledge [da'ath] (gnosis) and understanding [tevuna] (synesis)"
— Proverbs 2.6
Philo also refers to the "knowledge" (gnosis) and "wisdom" (sophia) of God.
The Church Fathers 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. 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 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) it is distinguished from ordinary epistemological knowledge (episteme—i.e., speculative philosophy).
Knowledge (or gnosis) in Sufism refers to knowledge of Self and God. The gnostic is called al-arif bi'lah or "one who knows by God". The goal of the Sufi practitioner is to remove inner obstacles to the knowledge of god. Sufism, understood as the quest for Truth, is to seek for the separate existence of the Self to be consumed by Truth, as stated by the Sufi poet Mansur al-Hallaj, who was executed for saying "I am the Truth" (ana'l haqq).
^A concise dictionary of theology by Gerald O'Collins, Edward G. Farrugia p. 130 Publisher: T. & T. Clark Publishers (2004) ISBN978-0-567-08354-8
^"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
^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.