Nobody in antiquity called themselves a Sethian in the sense the word now carries. The category was constructed in the twentieth century, principally by Hans-Martin Schenke, who noticed that a set of texts from the Nag Hammadi codices share a cluster of features: the unknowable one and Barbelo, the four luminaries, the ignorant craftsman, Seth as ancestor and revealer, and a rite of ascent described with the language of baptism. Texts assigned to the group include the Apocryphon of John, the Hypostasis of the Archons, the Gospel of the Egyptians, the Trimorphic Protennoia, Zostrianos, Allogenes and the Three Steles of Seth.
There is external corroboration, which is unusual for this material. Porphyry reports that in the school of Plotinus in Rome in the third century there circulated apocalypses under the names of Zostrianos and Allogenes, and Plotinus wrote a treatise against the people who read them. Two texts with those titles were recovered at Nag Hammadi, and the later ones among them are markedly more philosophical and less mythological, which suggests a tradition in conversation with Platonists.
The category is also contested. Michael Williams and Karen King have argued that Gnosticism as a class is a construct built out of the polemics of the church fathers and that it obscures more than it explains, and that the subgroups within it need the same scrutiny. Sethian has held up better than most because it rests on shared features in the texts themselves.