Hi!
The strangely later so-called church-fathers certainly had their own little agendas, some better
, some worse. As far as I understand Paul or John, they based their critique against 'gnostics' mainly on the ground of their ethics. The gnostic doctrines depreciated the physicality of life and death of this man, whom we deem to be the Christ of God. They fancied their ideas more than the bodily existence (of their fellow humans as well), not caring about their own and others' humanity, out of a contempt that was fuelled by haughty disregard for the actual perceivable reality.