Gnosticism is a theological error prevalent around the time of Christ. Generally speaking, Gnosticism taught that salvation is achieved through special knowledge (gnosis). This knowledge usually dealt with the individual's relationship to the transcendent Being. It denies the incarnation of God as the Son. In so doing, it denies the true efficacy of the atonement since, if Jesus is not God, He could not atone for all of mankind, and we would still be lost in our sins.
Please see Heresies for more information.