Gnosticism is a very complex system of religious doctrines and practices, philosophical, theurgical, and mystagogical in character, which began in the Alexandrian period in Judeo-pagan circles and developed in the first centuries of Christianity.
The basic principle of the "gnosis’ is: In religion there is a common faith, which may be sufficient for the ordinary people, but there is also a higher knowledge, reserved to the learned, which offers a philosophical explanation of the common faith. Christian Gnosticism draws various elements from Plato, from Persian Mazdaism, from pagan mysteries, and applies them to the Christian religion by using and abusing allegorical exegesis of Holy Scripture. The Christian “gnosis” may be defined as a theosophic philosophism tending to absorb divine revelation in order to make a religious philosophy of it. It developed in Syria with Simon Magus, Menander, and Saturninus, and in Egypt (Alexandrian gnosis) with Basilides, Valentine, and their respective disciples. Despite differences, the “gnosis” is reducible, more or less, to this outline: (a) God is an inaccessible Being (Platonic transcendence), who can have no contact outside of Himself; opposed to God but coeternal with Him is matter (Platonico-Persian dualism), bad in its nature (pessimism); (b) between God and matter is the pleroma or ogdoad, an intermediate, supersense world (the hyperuranium of Plato) inhabited by beings called aeons, emanating one from the other or disposed in pairs (syzygies); © one of aeons, the Demiurge (God of the Old Testament) worked matter into the actual form of this world; (d) a divine spark from that superior world fell one day on the matter of this world of ours and remained there to suffer as in a prison (soul in the body); (e) another of the aeons (Christ) descended into this world, took the appearance of a body (Docetism) and lived and died to free spirit from matter (Redemption); (f) side by side with these theories there was a moral teaching, often lax, and a superstitious cult, in which the sacraments appear deformed. Marcion developed some Gnostic elements along lines of a very predominant and austere asceticism.
Gnosticism constituted one of the gravest dangers for the newborn Christianity; Judaism was the other. Fortunately, Gnosticism was anti-Jewish. The Fathers spotted the menace immediately and endeavored to eliminate it. St. Irenaeus refutes Gnosticism in the 5 books of his “Adversus Haereses”. His position, like that of Tertullian, is conservative, with uncompromising reaction; but in Alexandria, Clement and Origin used the false gnosis to build up a Christian gnosis (science in service of the faith): hence theology was born.