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What Is Gnosticism?
Gnosticism is a term used by modern scholars to describe a variety of practices deemed by some early Christian leaders to be heretical. The Greek term gnosis means knowledge, but the term “Gnosticism” is employed most often to refer to false knowledge. According to most definitions, Gnostics believed that the creator god of the Hebrew Bible was an evil and ignorant power, and not the true God. A savior (Jesus) came into this world in order to teach people true knowledge (gnosis) about the nature of the created and uncreated worlds. Generally, Gnostics are understood to reject Hebrew Scripture, to embrace a dualistic theology in which there are good and evil powers controlling the world. They are also thought to have engaged in either ascetic or libertine practices.
Despite this negative judgment, it is important to keep in mind that ancient people who might fall under the category of “gnostic” would not have thought of themselves as deviant or in any way heretical. Early Christian “heresiologists” (Christians who wrote treatises denouncing beliefs and practices that they considered heretical) argued that gnostics (among others) imported ideas from the outside culture which poisoned the pure tradition of Christian faith stemming from Jesus. 
However, evidence shows that a concept of Christian orthodoxy was never present in the earliest Christian centuries. Instead, after Jesus’ death, there were many groups of Christians, each trying to interpret the meaning of his life and death, and those groups often differed radically. Calling someone “Gnostic” was a way for one Christian thinker to denounce another. 
In 1945 a trove of documents called the Nag Hammadi Codex was discovered in Egypt. This provided, for the first time, writings by groups that would have been condemned as heretical (though they're not, as "The Da Vinci Code" claims, the oldest Christian sources we have). These documents, including The Apocryphon of John, The Gospel of Truth, and The Gospel of Mary, show theological and ethical ideas at odds with the way the heresiologists described the gnostics. Many contemporary scholars are rethinking the category of “Gnosticism” in the light of this valuable new material.  
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Were There Gospels Aside From the Four in the Christian Bible?
Yes. The word gospel means “good news.” In the years following Jesus' death, stories about his teachings--and the meaning of his life and death--were transmitted orally. At some point, different members of the early Christian Church began writing down their versions of these stories and teachings, and it was not until the middle of the second century that we hear of disputes over which gospels should be accepted as authoritative. Even then, it took another few centuries before the Church decided what would be in the official list (canon) of the New Testament. 
The non-canonical gospels provide a window into the diversity of early Christian belief and practice, and show that there was a level of variety among early Christians of which many contemporary people are unaware.
Here are some of the gospels that did not make it into that canon:
"The Gospel of Thomas": This gospel is mostly a list of sayings attributed to Jesus. It has almost no narrative. The text claims that these secret sayings of Jesus were recorded by a man named Didymos Judas Thomas. While some of the sayings in this gospel are also found in the canonical gospels, others are unique. For example: “Jesus said: Become passers-by.” (Gospel of Thomas 42). In general, this gospel shows Jesus urging his disciples to remove themselves from the world and to seek wisdom: “Jesus said: Those who seek should not stop seeking until they find” (Gospel of Thomas 2).
"The Gospel of Mary": This gospel begins by telling of a resurrected Jesus, who is preaching to the apostles. His teaching emphasizes inner, spiritual knowledge. Jesus encourages his disciples not to get mired in the physical passions. After the preaching is finished, an argument ensues between Peter and Mary Magdalene over who is the best disciple, and who understood most about Jesus’ teaching.
"The Infancy Gospel of James": This text contains the story of the miraculous birth and childhood of Mary, the mother of Jesus. According to this gospel, Mary was raised in the Jerusalem Temple. Upon reaching puberty, she is sent to live with Joseph, who refuses to marry her, but agrees to be her guardian. The story continues with Jesus’ conception and birth, following the basic narrative in the Gospel of Matthew, with a few differences.
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