Mandaeism
According to the Fihrist of ibn al-Nadim, Mani, the founder of Manichaeism, was brought up within the Elkasaites (Elcesaites or Elchasaite) sect. The Elkasaites were a Christian baptismal sect which may have been related to the Mandaeans. The members of this sect wore white and performed baptisms like the Mandaeans. They lived in east Judea and northern Mesopotamia. According to the Harran Gawaitā legend, the Mandaeans migrated to southern Mesopotamia from there. Mani later left the Elkasaites to start his own religion. Mandaean scholar Säve-Söderberg showed that Mani's Psalms of Thomas were closely related to Mandaean texts. This would imply that Mani had access to Mandaean religious literature.