The Valentinians

The school of Valentinus — the most intellectually formidable and the most CHURCHY of these currents. They had sacraments, congregations, teachers and students, and for a long time they worshipped inside ordinary churches while holding a deeper reading privately.

The reason the boundary between "orthodox" and "Gnostic" was, at the time, porous and contested rather than obvious. Irenaeus's complaint is partly that you cannot tell them apart on a Sunday.