Upon his death he was declared a god by the imperial court and subsequently known as Tosho Daigongen ( The Great Incarnation Who Illuminates the East ).
He became known as a god after his death because of the many great deeds he did during his lifetime. 
mnli hypothesis: He became known as a god after his death because of the many great deeds he did during his lifetime.  premise: Upon his death he was declared a god by the imperial court and subsequently known as Tosho Daigongen ( The Great Incarnation Who Illuminates the East ).