Nehan-gyo (the Nirvana Sutra), has succeeded the idea of Kuon-jitsujo (Buddha had attained Enlightenment in the eternal past) from Hokke-kyo Sutra, and developed and sublimated it into an idea that the real nature of Budddha shall exist forever even after Nyorai (Buddha) entered nirvana.
なお涅槃経では、法華経の久遠実成を引継ぎつつ、それを更に発展昇華し、如来は入滅後においてもその仏性は永遠に存在すると説いた。