首楞嚴義疏注經 7.53

7.53

將說神呪現光化佛，化復作化百河沙眾。此即如來藏心不思議妙用，一即一切、一切即一。大眾將證此理，故示現以表也。下說神呪是此密因，宣與大眾化他令益。

About to speak the divine mantra, he manifested light and a transformation buddha, and the transformation further manifested a hundred Ganges sands of beings. This is the inconceivable wondrous functioning of the tathāgatagarbha mind: the one is the all, and the all is the one. Because the great assembly was about to realize this principle, it was manifested to symbolize it. The divine mantra spoken below is this secret cause, proclaimed to the great assembly to transform others and bring them benefit.