大毘盧遮那成佛經疏 11.21

11.21

「何以故？於無量劫勤求修諸苦行所不能得，而真言門行道諸菩薩即於此生而獲得」，如餘菩薩為求無上道故，難行苦行如救頭燃，經無量劫尚不能得如此成就。此真言行者若具方便，聖可成就也。以知甚深緣起十喻義故，以真言等願作種種成就而無取著，不異於如是法微妙速疾也。

Regarding 'Why is this? That which could not be obtained by diligent pursuit and cultivation of various austerities over immeasurable kalpas, bodhisattvas practicing the path of the mantra gateway obtain right in this lifetime': other bodhisattvas, for the sake of seeking the unsurpassed path, undergo difficult and austere practices as if rescuing their heads from burning, yet after passing through immeasurable kalpas still cannot obtain such an accomplishment. If this mantra practitioner is endowed with upāya, [such an accomplishment] can indeed be achieved. Because they know the meaning of the ten metaphors for profound dependent origination, through mantras and vows they bring about various accomplishments without attachment, which is no different from the subtlety and swiftness of such Dharma.