法華玄論 9.38

9.38

答：不光照而忽集，即是當時化作，而十方無有實佛，便不得顯迹多本一義也。又若不放光照，即無因緣，故佛不得集；今會集佛，必有因緣也。

Answer: If they gathered suddenly without the light shining, they would merely have been conjured up at that moment, and there would be no real buddhas in the ten directions, making it impossible to reveal the meaning of manifold traces and a single root. Furthermore, if light were not emitted, there would be no causes and conditions, so the buddhas could not gather; now, when the buddhas gather, there must be causes and conditions.