大毘盧遮那成佛經疏 10.128

10.128

時毘盧遮那佛自教迹不空悉地樂欲一切菩薩母明妃說者，自教迹者，即是法佛自證之教，即祕密平等教也。為於此中諸為修行者皆悉不空，不空是不唐捐義。隨彼力能皆向法身之理，即同彼佛，故云不空也。如上諸菩薩說真言，各欲引攝同類行者，若有修行即同於我。今法佛自說明妃真言，若有修行者，即從虛空眼而生法身，如我無異也。

As for when Vairocana Buddha speaks the vidyārājñī, the mother of all bodhisattvas who grants unerring siddhi and desires from his own teachings: "His own teachings" refers to the teachings realized by the Dharma Buddha himself, namely the secret and equal teachings. It is so that all practitioners herein will not be in vain—"unerring" means not being in vain. In accordance with their capacity, they all tend toward the principle of the dharmakāya and become identical to that Buddha; therefore it is said to be "unerring." Just as the bodhisattvas mentioned above spoke mantras each wishing to guide and gather practitioners of their own kind, such that if anyone practices, they will become equal to them; now the Dharma Buddha himself speaks the vidyārājñī mantra, so that if anyone practices it, they will be born into the dharmakāya from Gaganarocanā, no different from myself.