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.