This is based on the idea of kubon-ojo (the nine classes of people reborn in the Land of Happiness according to the nine levels of religiousness), as preached in 'Kanmuryoju-kyo sutra' (The Sutra of Visualization of the Buddha of Measureless Life, meaning Amida).
これは「観無量寿経」に説く九品（くぼんおうじょう）の思想に基づくものである。