This book introducing the Gnostic literature is an excellent introduction to an ancient derivative of Christianity, probably originating in the days of Jesus. The Gnostic literature and its adherents were later condemned as heretical and destroyed by the Second Century Christian Church. This hostility may well have originated with the Apostle Paul in the First Century who claims he got all his information directly from Jesus after Jesus' death by crucifiction. Paul had never talked with Jesus or his followers before Jesus' death: "But I make known to you, brethren, that the gospel which was preached by me is not according to man. For I neither received it from a man, nor was I taught it, but it came through the revelation of Jesus Christ" (Galations 1:9-10). On the other hand, he was not in favour of other people's revelations or understanding: "O Timothy! Guard what was committed to your trust, avoiding the vain and idle babblings and contradictions of what is falsely called knowledge [gnosis - Greek] - by professing it some have strayed concerning the faith" (1 Timothy 6:20-21). Paul was equally hostile to the early followers of Jesus who continued to adhere to the Law of Moses. Because the adherents of Gnosis or Gnosticism and their literature were destroyed, the only information about them was that provided by their persecutors, until the discovery of the lost texts in 1945 C.E.
Elaine Pagels provides a detailed analysis of the history and contents of the Gnostic literature.
Quotes are from: Holy Bible, The New King James Version, 1990