The father of Gnosticism was Simon Magus (Acts 8). Two major schools had developed by 150 AD, the followers of Valentinus and the followers of Basilides. Our knowledge of Gnostics comes mainly from the early church father Irenaeus, who wrote the five volume set "Against Heresies" and the Gnostic literature itself found at Nag Hammadi, Egypt in 1945. Irenaeus states that Gnostics derived their teachings from the Heathen, namely Homer, Plato, Aristotle, the Pythagoreans and others.¹⁶  John the Apostle would not enter a bathhouse where Cerinthus was, and Polycarp said Marcion was the first born of Satan.¹⁸

The basic teachings of Gnosticism that the church called heresy are:

1. There are 30 Aeons (Gods) that exist in the Pleroma, outside of time and space. ¹ 
2. The Goddess, Sophia, created the Demiurge, a creator angel (the god of the old testament) who was a tyrant and being unaware of the Aeons, thought he was the only God. He created man but Sophia gave man a spirit. ² 
3. Some may be saved if they do enough good works but some are predestined to go to hell. ⁴  (works salvation)
4. Gnostics have spirits that are emanations from Sophia, this makes them predestined to be saved. It is imposable for them to loose their salvation. It does not matter if they act good or evil. So the most "perfect" of then addict themselves to evil deeds and are in a habit of defiling the women they convert. ³ 
5. Eventually all mater will be destroyed being evil and not capable of salvation.. ^{9, 6}
6. Gnostics will become spirits and will marry the angels. ⁹
7. Christ descended upon Jesus at his baptism and left before Jesus went before pilot. Sophia would not allow Christ to suffer. ⁹ (Adoptionism)
8. They utter mantras to effect nature. ¹⁰ (Hindu mantra and Kabalistic letter magic, Gramera, and emanations)
9. Souls reincarnate. ³²
10. Perfect knowledge is obtained by baptism, spiritual marriage and last rites. ⁵  (sacramentalism)
11. Sophia sent the serpent (the angel Michael or Samael) into the garden of Eden to free Eve and Adam. By eating of the tree they attained true Gnosis and were set free. ¹⁵
12. Sophia saved Noah from the flood of the evil Demiurge. ¹⁵
13. The Demiurge forced Eve into sexual intercourse many times. Eve thereby, gave birth to other evil creator angels. ¹⁵ (serpent seed)
14. At death the some souls enter an intermediate state to be purged of the animal nature before going into the Pleroma. ¹⁷ (Purgatory)

Later other Gnostics would add or change some points and found their own school of Gnosticism.

• Simon Magus - 
  • Feigned faith, and used exorcisms and incantations, love-potions, and charms, as well as those beings who are called "Paredri" (familiars) and "Oniropompi" (dream-senders). ⁶
  • He allegorized much scripture to support his teachings. ⁴⁴ especially Genesis - ⁴⁵
• Menander - By means of magic one may overcome the angels that made the world. Only if you are baptized into Menander you will obtain the resurrection and never die again having eternal youth. ⁶
• Ebionites -
  • God created the world but Jesus was just a man and use the gospel of Matthew only and say Paul was an apostate and follow the Jewish customs. ^{12, 38}
  • Founded by Ebion ³⁸
• Nicolaitanes -
  • Practice adultery, and to eat things sacrificed to idols. ¹²
  • Became very obscene and Gnostic. ³⁷
• The Carpocrates -
  •  Say Nicolas and Mathias taught fornication is no longer wrong. ²⁴
  • We are imprisoned in a cycle of reincarnation by those evil creator angels, but will eventually break the cycle and be saved. ⁸
  • Was a magician and a fornicator. ⁴⁰
  • Carpocrates practice their magical arts and incantations, and spells and voluptuous feasts. And (they are in the habit of invoking the aid of) subordinate demons and dream-senders. ⁴⁹
• Cerinthus - Jesus was just a man and the Christ descended on him at this baptism and departed before he suffered on the cross. ¹² (Adoptionism)
• Saturninus ⁷ - 
  • He, himself, was and angel. ⁷
  • Jesus did not have a physical body. ⁷
  • Jesus came to destroy the god of the Jews. ⁷
  • Sex, marriage, and reproduction are sinful. ^7,14
  • Saturnilus and his school were vegetations. marriage and procreation are from Satan. They practice asceticism ⁴⁸
• Marcus -  
  • Taught that the Holy Spirit put a drop of her blood into the wine when he blessed it. Upon drinking it you would understand mysteries and prophesy. (transubstantiation) ¹¹
  • If anything bad begins to happen to them they pray to the mother who raptures them.¹¹
  • Marcus and the cup that imparts grace. the prayer Marcus used. ⁴⁶
  • Marcus teaches a second baptism (of redemption) this a lying on of hands and a word or phrase given by the bishop Marcus only when he is ready to go on to the higher mysteries, or when the disciple is dying. They are taught to keep secret the word and even deny that is exists. (last rites) ⁴⁷
• Marcion - 
  • Rejects the Old Testament and uses cut up versions of Luke and some of Paul's epistles. The god of the old testament and his prophets and are evil and will be destroyed. ¹³
  • God is the author of sin. ²⁷
  • There are two equal and opposite gods one good and one evil. ²⁸
  • Strongly addicted to astrology. ²⁹
  • The Law and The Gospel being so against each other proves two different gods. ³⁰
  • There is no resurrection. The saved should not marry. ³¹
  • Removed references in his gospel that Christ was creator ³³
  • Jesus was a phantom (no physical body) ³⁴
• Titian - 
  • Adam was not saved. ¹⁹
  • Drinking wine is a sin. ²²
  • The soul is not eternal, it dissolves with the body. ²⁰ (soul sleep)
  • Medicine is demonic, instead of taking it, we should rely on God alone. ²¹
• Ophites - Worshiped the serpent as primary Eon (brazen). ³⁸
• Cainites -Worship Cain the brother of Abel as primary Eon. ³⁸
• Cerdo - First to introduce dualism. ³⁹
• Apelles - Agrees Jesus was flesh, but after the resurrection he disintegrated it. ⁵⁰
• Calistus - Formed a school (commune) that allowed fornication, common law marriages, the use of drugs for producing sterility, abortions (called murder by him), no true Christian does these things. ⁵¹
• Alcibiades (Elchasai) - Formed the Elchasaites who taught there are female angels, a new remission of sins (based on the teachings of his book) teachings were similar to the teachings of Callistus, Incantations, circumcised, Christ was born a man, reincarnation, astrologers. ⁵²
• Elchasaites - Jesus incarnated many times, resort to incantations and baptisms in their confession of elements. And they occupy themselves with bustling activity in regard of astrological and mathematical science, and of the arts of sorcery. But also they allege themselves to have powers of prescience ⁵³
• Naasseni -Worship the serpent; teach their order was started by James the Lord's brother, Adam was a hermaphrodite, they use Gospel According to Thomas, some things taken from the Mysteries of Isis and Mysteries of the Assyrians, and practice orgies. ⁴¹
• Peratae -
  • Teach a kind of tri-theism. ⁴²
  • Another branch is the Peratae, who teach Jesus the son is the serpent. ⁴³
• Basilides - 
  • Reincarnation with Karma ²⁵
  • Man can grow up and be sinless all of his life. ²⁵ (Born without original sin)
  • Spirits (sometimes animal spirits) latch on to us and force us to sin. ²³
  • Saving Faith is intellectual ascent. All humans are born with the ability. ²³ (Pelegianism)
  • After baptism, God forgives involuntary sins but you must pay for all voluntary sins in order to be purged from them. ²⁶ (penance, purgatory, cardinal and venial sins)
  • Jesus was transformed to look like Simon of Cyrene, Simon was crucified in his place. ^{7, 37}
  • Supreme god is Abraxas, ³⁷
• Valentinus - 
  • Saving Faith comes from your spirit, if you are one of the chosen. ²³ (born saved)
  • Only those who's spirit is an emanation from Sophia are predestined to be saved. All others are predestined to hell. ²³ (double predestination)
  • Some are not predestined to be saved, are predestined for hell but the rest might be saved through works. ³⁵
  • The predestined for salvation do not need to practice good works. ³⁶

In combating Gnosticism the scripture says the following:

• 1 John 1:5 There is no duality in God.
• 1 John 2:23 Whoever says Jesus is not the Christ is of antichrist.
• 1 John 4:3 Whoever says Jesus did not come in the flesh is of antichrist. 
• 2 John 7,9-10 Do not welcome or allow into your house anyone who says Jesus did not come in the flesh.

Lactantius in, Divine Institutes 4.30 says:
When they are called Phrygians, Novatians, Valentinians, Marcionites, Anthropians, or Arians they have ceased to be Christians.