CC Madhya 6.82

anumāna pramāṇa nahe īśvara-tattva-jñāne
kṛpā vinā īśvarere keha nāhi jāne
Synonyms: 
anumāna pramāṇa — evidence by hypothesis; nahe — there is not; īśvara-tattva-jñāne — in understanding the Absolute Truth, the Supreme Personality of Godhead; kṛpā vinā — without His mercy; īśvarere — the Supreme Personality of Godhead; keha — anyone; nāhi — not; jāne — knows.
Translation: 
Gopīnātha Ācārya continued, “One can understand the Supreme Personality of Godhead only by His mercy, not by guesswork or hypothesis.”
Purport: 

One cannot understand the Supreme Personality of Godhead simply by exhibiting some mundane magic. Foolish people are enchanted by magical demonstrations, and when they see a few wonderful things done by mystical power, they accept a magician as the Personality of Godhead or an incarnation. This is not the way of realization. Nor should one guess or speculate about an incarnation of God or the Personality of Godhead. One has to learn from the bona fide person or from the Supreme Personality of Godhead Himself, as Arjuna did, by the mercy of Kṛṣṇa. Kṛṣṇa Himself also gives many hints about His potencies as the Supreme Personality of Godhead. One should understand the Supreme Personality of Godhead only through the evidence presented by the śāstras and the mahājanas. In any case, one must have the mercy of the Lord in order to understand the Supreme Personality of Godhead by devotional service.