Why does Gnosticism pose a threat to Christianity?
In fact, Gnosticism was not so much a threat as it was considered heretical, and therefore a danger to those who practiced it. Gnosticism was based on the teachings of the pre-Christian Greek philosophers. Gnostic Christians attempted to assimilate Greek philosophical teachings with those of the early church. It is perhaps best illustrated in the Gnostic Gospels, long deleted from Scripture, including the Gospel of Mary, the Gospel of Judas, and the Gospel of Thomas. Gnosticism taught that salvation came from mediation, not through the work of the Church. More importantly, the Gnostics believed that Jesus never took human form (contrary to the above response, which is incorrect.) He was wholly divine, and thus appeared only as a spirit. He presumably cast no shadow, and left no footprints when he walked. Part of this belief originated from the idea of the sinful nature of humanity; since Jesus was without sin, he could not be human, and was therefore wholly spiritual. This teaching was directly contrary to the teaching of the church adopted at the Council of Nicaea which stated that Jesus was both human and divine. This was heresy, which condemned its believers to hell. Thus it was more a threat to its followers than to the church hierarchy itself.
Gnosticism no longer poses any real threat to orthodox Christianity, but it was more of a threat in early Christian times.
In temporal terms, it posed a threat to early orthodox Christianity because it had many followers. This took power and influence away from orthodox Christianity. It also disputed the idea that there should be an institutional church with a hierarchy. Instead, it preferred mysticism and the passing on of esoteric knowledge to believers.
In theological terms, it posed a threat to early orthodox Christianity because it has a much different world view. To the Gnostics, the entire physical universe is a mistake made by a being who is obviously very powerful, but is not exactly the creator God. Many Gnostics also held to the idea that Jesus Christ was not divine. These sorts of beliefs were clearly a threat to Christianity in the sense that they were completely opposed to Christian teaching.