Prologue

Gnosticism is a dualistic belief: the spirit is good, the matter is evil.
Salvation isn’t by faith in the resurrection of Jesus, but by a ‘secret knowledge’ (the Greek word ‘gnosis’ for ‘knowledge’ in English).

Many Gnostic sects existed, and they all have their own sets of doctrines.
These Gnostics sects are also the secret authors of the controversial Nag Hammadi library.

Cosmology

Demiurge.

Origins

Pre-Christian era

The most ancient root of Gnostic idea is unclear, but it certainly contains some influences of Plato. The Greek philosopher Plato (428 – 348 BC), argues the duality of the soul (good) & the body (bad) on his book “Phaedo“. The story begins with the influence of Platonism in the early Church.

Early Church (1st century)

Simon Magus

Simon Magus (the magician) is believed to be the founder of Gnosticism.
He appears in Acts 8:9-25, where he confront Simon Peter. It’s pretty interesting, you have 2 Simon: one is the head of the Catholic Church, the other is the head of Gnosticism, it’s light versus darkness.

Cerinthus & the Nicolaitans seems to appear in the 1st century, where they confronted John the Apostle. That’s the reason why John wrote his Gospels and letters, where he put emphasis on the fleshly nature of Jesus.

The Cathars are a post-gnostic sect.

Gnostic dualistic theology started to appears in the early 2nd century, influenced by the concept of docetism, under Marcion (85 – 160 AD).

At the time of the Apostles, the docetists (Greek word that mean ‘to seem / appear’) who believed that Jesus never existed in a physical form, but only in a spiritual form.

Marcion (85-160 AD) was the 1st known theologian to develop the idea of a spiritual Jesus, not a fleshly one. He study under Cerdo, who then was himself a student of the sorcerer Simon Magus (Acts 8:9-24).
Simon Magus is believed to be the founder of Gnosticism.

And where did that idea started from? The Greek philosopher Plato.

(the material world is evil).

Since Marcion thinks that the material world is evil, it’s not possible for him that a good God would ever want to be in the flesh.

Clearly the early Church were against the docetists, and to refute them, John started his Gospel with the famous passage to indicate that Jesus was truly human.

“And the Word became flesh and made his dwelling among us”
John 1:14

Christians has always believed in the importance of the material world. In Genesis, everything that God created (including the material world) was good. Otherwise, sacraments would be useless.

That is why docetists reject the literal historicity, birth, death & resurrection of Jesus.