You'll find that in the Bhagavad-gītā, when Arjuna was convinced that Kṛṣṇa is the Supreme Personality of Godhead... But because in future others will have doubt about Kṛṣṇa, he requested Kṛṣṇa that "Will You show me Your universal form?" And Kṛṣṇa agreed and showed him the universal form.
Śrīla Prabhupāda's lecture on Bhagavad-gītā 2.11 -- New York, March 4, 1966
