法華論疏 2.91

2.91

何故甚深者，謂大菩提故。大菩提者，如來所證阿耨多羅三藐三菩提故。又甚深者，一切聲聞、辟支佛等所不能知，故名甚深。

For what reason is it profound? Because of great bodhi. Great bodhi refers to the anuttarā-samyak-saṃbodhi realized by the Tathāgata. Furthermore, 'profound' means that which all śrāvakas, pratyekabuddhas, and so forth are incapable of knowing; therefore, it is called profound.