The Heresy of the Judaizers
By tradition, Gennadius first became aware of the Judaizing movement in 1487, when its existence was disclosed by adherents who had revealed themselves while intoxicated. The heresy, which had taken root in Novgorod and spread to Moscow, was reported by later sources to promote anti-Trinitarian teaching. Gennadius reported his findings up the hierarchy and devoted much of his episcopate to combating the movement.
He worked in close concert with Saint Joseph of Volokolamsk, the monastic reformer, and councils were convened to address the sectarians. The synaxarion records that he labored against the heresy for some twelve years.