大般涅槃經義記 1.131

1.131

問曰：真常昔來隱覆，今可須開，應是舊化，昔來恒見，何勞說乎？釋言：應身雖是舊化，昔來言實，今方言應，是故須辯。又復權隱導引化廢，故須明應。明佛真常，欲令眾生趣入無為；顯佛應滅，為使眾生同厭有為。

Question: The true and eternal was formerly concealed and now indeed needs to be opened; the response body is the old transformation and has always been seen in the past—why labor to speak of it? Explanation: Although the response body is the old transformation, in the past it was called real, and only now is it called a response; therefore it must be discerned. Furthermore, provisionally concealing it to guide [beings] rendered that transformation obsolete, and therefore the response must be clarified. Clarifying the Buddha's true eternity is intended to cause sentient beings to enter the unconditioned; revealing the Buddha's responsive extinction is done to make sentient beings equally detest the conditioned.