I've recently re-absorbed the last chapter of The Gnostic Gospels by Elaine Pagels, making extensive notes in the process. This chapter is entitled "Gnosis: Self-Knowledge as Knowledge of God" and gets to the essence of what I find appealing in gnostic thinking. Let me try to integrate what I've been reading....
First, it's not quite clear who the gnostics were. Pagels concentrates on gnostic Christians, but it's possible that gnosticism pre-dates Christianity or that gnostic Christians represented a merging of a separate gnostic tradition (perhaps within Judaism, perhaps not) with the then-new Christian tradition. Be that as it may, gnostic Christianity was opposed to the claims of the catholic church that it was the true orthodox (right-thinking) branch of Christian tradition. The gnostics did not accept the authority claimed by the clergy, nor did they accept the books codified into the New Testament as the true teachings of Jesus. The catholics asserted that "outside the church there is no salvation", whereas the gnostics held that salvation comes not through an institution but through the cultivation of personal wisdom. For the gnostics, Jesus was a teacher of wisdom, not the cleanser of sins. Divinity is within the individual, not utterly foreign to human experience. The gnostic is an often-solitary seeker after insight (in Greek, a monachos, from which term come "monk" and "monastic"). Yet the gnostic is not a hermit, but a member of a community of fellow-seekers. But the true religious community, for the gnostics, is measured not by obeisance to clerical authority or by recitation of a certain creed, but by the extent of the knowledge and wisdom gained by its members. What matters is not expiation of some original sin, but an ascent from ignorance to insight that is captured in the phrase "spiritual maturity".
And that spiritual maturity is reached through the search for self-knowledge. The gnostics held that far from requiring a church or divine revelation, the human individual possesses a full capacity for liberation from ignorance and unconsciousness. Certainly one experiences internal resistance in that search for enlightenment, but that resistance can be overcome. And to overcome it is necessary in order to "become what you are"; the choice is liberation or destruction, light or darkness: "If you bring forth what is within you, what you bring forth will save you. If you do not bring forth what is within you, what you do not bring forth will destroy you." (A saying attributed to Jesus in the Gospel of Thomas.) And also: "There is a light within a man of light, and it lights up the whole world. If he does not shine, he is darkness." (ibid.) And what is this light within that one must cultivate? "The lamp of the body is the mind", according to a saying of Jesus in the Dialogue of the Savior. The gnostic searcher is exhorted to "light the lamp within you" and to "knock upon yourself as upon a door and walk upon yourself as on a straight road" (Silvanus). "Whoever has not known himself has known nothing, but he who has known himself has at the same time already achieved knowledge about the depths of all things" (Book of Thomas the Contender). The "Kingdom of God" is a state, not of history, but of transformed awareness.
Thus self-knowledge is the key to understanding the divine. Indeed, according to at least some gnostics (especially followers of Valentinus), human beings created the language of divinity -- even created god in their own image! Religious language is not literal, since it does not refer to separate objects or entities out there in the world; instead, it is a "language of internal transformation" in which "you see yourself, and what you see you shall become" (p. 134). The Jesus of the gnostic gospels directs his disciples inward. For example, in the Dialogue of the Savior, Matthew asks Jesus to show him the "place of life" which is "pure light", and Jesus answers that "Every one of you who has known himself has seen it." The disciple -- any disciple -- who seeks the truth is also the one who reveals the truth; for the truth is within.
One implication of finding truth within is that external authorities such as the church or even the reports of the students of Jesus (the apostles, in the terms of the catholic church) are no longer necessary -- the true disciple discovers that his own mind "is the father of the truth" and thus "maintains his own independence of anyone else's authority" (p.132). "No one else can tell another which way to go, what to do, how to act" (p.145). Such an attitude was, for obvious reasons, unpopular with those who would set themselves up, through the doctrine of apostolic succession, as the only authority in matters of the spirit.
It comes as no surprise, then, that a gnostic approach to Christian thought did not survive: it stood as a direct challenge to the emerging orthodoxy. Yet in a sense gnosticism, however individualistic and true, contained within itself the seeds for its own repression. Why? For the very reason that it was individualistic and true. It strikes me that a tradition stressing spiritual maturity is by its nature elitist in some fashion. It is an approach for the few. Further, Pagels argues that gnostics emphasized the divinity of human nature to such a degree that, as Plotinus said, they thought "very well of themselves, and very ill of the universe". Though I take anything Plotinus says with a large grain of salt, I can see some truth here: the gnostics tended to pursue a fairly solitary life of contemplation, sometimes to the detriment or exclusion of all engagement with the world through marriage, parenthood, work, and community involvement. I see parallels here to the contrast between, say, Aristotelianism and Epicureanism, or Confucianism and Taoism. I feel that this tension (not to say "conflict", at least not necessarily) derives from the very nature of the human individual as what the anthropologists call a "social solitary". Few if any philosophers have, I think, done full justice to the fact that we are social solitaries. Partly that's because there is such a large grey area here. But partly there is here also a tendency to totalism -- to saying that it is only the solitary life of the individal that truly matters (Epicureans, gnostics, Taoists, Randians) or to saying that it is only the social life and progress of humanity that truly matters (early catholics Christians, Confucians, socialists). Yet these are merely tendencies, and within each tradition one can find thinkers who seek a greater integration between the solitary and the social. To me this is one of the foundational issues of human existence, since this duality has been present within us for millions of years as we evolved from merely the third chimpanzee into homo sapiens.