As with any other resilient heretical or erroneous doctrine, there is a kernel of truth at the heart of Gnosticism: namely, that if you are *merely* worldly, then the world is indeed truly evil, and with it the whole of our existence in it. By itself the world cannot but redound to its own corruption and eventual certain dissolution, rendering all creaturely suffering endured along the way completely pointless, base, and stupid. Mere worldliness is no more than ugly death.
The world and our life in it can be good only insofar as we approach it sub specie aeternitatis. In the world, but not of it; that’s the ticket.
So anyone who enjoys life partakes willy nilly of its divine archetype, no matter what doctrines he ostends, or intends. Glad hearts cannot be nihilist, despite their protestations to the contrary. But some there be who are so determined upon their philosophy as to gut all their enjoyments and evacuate all mundane meanings. Such are philosophical Gnostics.
The whole effort of the Gnostic is to climb up out of his own mere finity into infinity, so as to heal the ugly breach in his own being. This he tries to do under his own steam, either by rising up the Great Chain of Being from strength to strength, or else by eliminating his own being one way or another. The former method cannot work, for finity is inadequate to infinity by definition. So generally Gnostics sooner or later settle upon nihilism and the elimination of the self and its inherently limited and thus in so far forth delusory notions.
There is another way, which is the genius of Christianity: to invite God into the created order – or rather, to remember that he is there to begin with, its source and end – and specifically into the Christian gnostic’s own heart, there to take up kingship and reign. Only infinity is adequate to itself; indeed, only infinity is adequate even to finity. So only by means of Incarnation can the rottenness of the created order be redeemed, and made worthy. Only then by means of participation in that Incarnation may the gnostic himself be redeemed and healed of his disease, so that he may then turn and simply enjoy life as he was originally meant to do before creatures got themselves busy with the creation, and so ruined it.