Here is a great introduction to Gnosticism. My friend Kegan Chandler, over at his blog Buried Deep, has a golden pen, so to speak. It would be worth your time to check-out some of his recent posts. Also, tomorrow (August 24, 2016) is the launch of his new book "The God of Jesus in Light of Christian Dogma: The Recovery of New Testament Theology."
"First, what do we mean by “Gnosticism”? Today the term describes a fluid category of spiritual traditions developing in the mid to late first century CE which emphasized the acquisition of gnosis, or secret knowledge of the divine, in order to achieve salvation. While the movement was not exclusive to Christianity, it is best known by its manifestation in various Christian groups deemed “heretical” by the great proto-orthodox polemicists of the second and third centuries. . . . Gnosis-inclined “proto-orthodox” Christians, like Clement of Alexandria (d. 215 CE) and Origen (d. 254 CE), also employed a hermeneutic which assumed a spiritual meaning behind the text reflective of real happenings in a real spiritual world. This affinity may provide a clue to a deeper relationship between the “Gnostic” and the emerging “catholic” Christianity of the third and fourth centuries than many have supposed"