CC Madhya 7.13

viśvarūpa-siddhi-prāpti jānena sakala
dakṣiṇa-deśa uddhārite karena ei chala
Word for word: 
viśvarūpa — of Viśvarūpa; siddhi — of perfection; prāpti — achievement; jānena — the Lord knows; sakala — everything; dakṣiṇa-deśa — South India; uddhārite — just to liberate; karena — makes; ei — this; chala — pretense.
Translation: 
Knowing everything, Śrī Caitanya Mahāprabhu was aware that Viśvarūpa had already passed away. A pretense of ignorance was necessary, however, so that He could go to South India and liberate the people there.