Before Constantine and the Council of Nicea (A.D. 325), there were various religions that followed Jesus Christ. One group of early Christians was called 'Gnostic'. They claimed to have experience of the divine, an experience they called gnosis ('knowledge').

Our word 'agnosticism' is based on the same Greek word for knowledge, with an alpha-privative (a-) meaning 'not'. 'Agnostics' do not know.

"The divine" that must be experienced to be a true follower of Christ, according to the Gnostics, is described as "authentic truths of existence." Their belief system is mystical and probably understandable only to those who have achieved some degree of gnosis, like Carl Gustav Jung, or theologians.

Some of the more concrete aspects of it are that (some) Gnostics believe the creator god is a type of demon; their god is masculine and feminine, with Mary Magdalene revered as a consort to Jesus; their sacraments are baptism, chrism, eucharist, redemption, and a bridal chamber; and their religious practices seem to blend elements of asceticism and promiscuity.
In A.D. 180, the Bishop of Lyons, Irenaeus, published attacks against the Gnostic "heresy". Such attacks continued through the next century. By the end of the fourth century, Gnostics had been ostracized and most of their sacred texts destroyed. Until the twentieth century there were only three codices containing Gnostic writing, the Bruce, Askew, and Berlin Codices, plus quotes from hostile mainstream Christian writers. The Askew and Berlin Codices contain what is known as Pistis Sophia, which contains the post-Resurrection teaching of Jesus to his disciples, who include Mary Magdalene, Martha, and Mary, the mother of Jesus. Then in 1945, near Nag Hammadi, Egypt, a group of farmers found a red earthenware jar stuffed with 13 papyri containing fifty-two sacred texts, written in Coptic (the language of the Egyptian Christians), from the Gnostic Gospels.

There was also a text of Plato's Republic. At 1600 years old, these are the oldest known books.

Sources: An Introduction to Gnosticism and The Nag Hammadi Library
The Nag Hammadi Library

The Nag Hammadi texts were written on papyrus in the form of a codex, rather than rolled into a scroll. A codex could be opened flat and written on both the front and the back of a page. A codex is the direct precursor of a modern book. Here are the contents of each papyrus or codex. The link below provides translations for most.

• Codex I (The Jung Codex)
  1 The Prayer of the Apostle Paul
  2 The Apocryphon of James
  3 The Gospel of Truth
  4 The Treatise on the Resurrection
  5 The Tripartite Tractate
  
• Codex II
  1 The Apocryphon of John
  2 The Gospel of Thomas
  3 The Gospel of Philip
  4 The Hypostasis of the Archons
  5 On the Origin of the World
  6 The Exegesis on the Soul
  7 The Book of Thomas the Contender
  
• Codex III
  1 The Apocryphon of John
  2 The Gospel of the Egyptians
  3 Eugnostos the Blessed
  4 The Sophia of Jesus Christ
  5 The Dialogue of the Savior
  
• Codex IV
  1 The Apocryphon of John
  2 The Gospel of the Egyptians
  
• Codex V
  1 Eugnostos the Blessed
  2 The Apocalypse of Paul
  3 The (First) Apocalypse of James
  4 The (Second) Apocalypse of James
  5 The Apocalypse of Adam
  
• Codex VI
  1 The Acts of Peter and the Twelve Apostles
  2 The Thunder, Perfect Mind
  3 Authoritative Teaching
  4 The Concept of Our Great Power
  5 Plato, Republic 588A-589B
  6 The Discourse on the Eighth and Ninth
  7 The Prayer of Thanksgiving
  8 Asclepius 21-29
  
• Codex VII
  1 The Paraphrase of Shem
  2 The Second Treatise of the Great Seth
  3 The Apocalypse of Peter
  4 The Teachings of Silvanus
  5 The Three Steles of Seth
  
• Codex VIII
  1 Zostrianos
  2 The Letter of Peter to Philip
  
• Codex IX
  1 Melchizedek
  2 The Thought of Norea
  3 The Testimony of Truth
  
• Codex X
  1 Marsanes
  
• Codex XI
  1 The Interpretation of Knowledge
  2 A Valentinian Exposition
  2a. On the Anointing
  2b. On the Baptism A
  2c. On the Baptism B
  2d. On the Eucharist A
  2e. On the Eucharist B
  3 Allogenes
  4 Hypsiphrone
  
• Codex XII
  1 The Sentences of Sextus
  2 The Gospel of Truth
  3 Fragments
  
• Codex XIII
  1 Trimorphic Protennoia
  2 On the Origin of the World

Source: The Nag Hammadi Library Codex Index
The Nag Hammadi texts have spurred interest in the role of women in the life of Jesus and in the early Christian Church. Was Mary Magdalene the chief disciple of Jesus, the "apostle of the apostles," or the mate of Jesus? Why do three of the Gospels of mainstream Christianity say she appeared at the Resurrection and spoke with Jesus or an angel, while Luke fails to name her? Why were women apparently pushed out of the role of apostle-priest? What forces determined which tradition won? Looking at the Gnostic documents provides support for a variety of perspectives, as the authors of Burstein and De Keijzer's Secrets of Mary Magdalene demonstrate.