1580s, "believer in a mystical religious doctrine of spiritual knowledge," from Late Latin Gnosticus "a Gnostic," from Late Greek Gnōstikos, noun use of adjective gnōstikos "knowing, able to discern, good at knowing," from gnōstos "known, to be known," from gignōskein "to learn, to come to know," from PIE root *gno- "to know." Applied to various early Christian sects that claimed direct personal knowledge beyond the Gospel or the Church hierarchy; they appeared in the first century A.D., flourished in the second, and were stamped out by the 6th.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
Others are reading
Definitions of Gnosticism from WordNet
a religious orientation advocating gnosis as the way to release a person's spiritual element; considered heresy by Christian churches;