It is stated in 7th Canto, 15th chapter, 26th and 27th verse of Bhagavat Mahapuran.
यस्य साक्षाद्भगवति ज्ञानदीपप्रदे गुरौ । मर्त्यासद्धी: श्रुतं तस्य सर्वं कुञ्जरशौचवत् ॥
Yasya sākṣhād bhagavati gyān-dīp-prade gurau. Martyāsad-dhīḥ śhrutaṁ tasya sarvaṁ kuñjara-śhauchavat.- Bhagwatam 7.15.26
एष वै भगवान्साक्षात् प्रधानपुरुषेश्वर: । योगेश्वरैर्विमृग्याङ्घ्रिर्लोको यं मन्यते नरम् ॥
Eṣha vai bhagavān sākṣhāt pradhān-puruṣheśhvaraḥ. Yogeśhvarair vimṛigyāṅghrir loko yaṁ manyate naram.- Bhagwatam 7.15.27
“Know such an ‘āchārya’ (spiritual teacher) to be ‘sākshāt Bhagwān’ (God in person). Do not apply human intelligence, O souls.”
If you apply your human mind (meaning having doubts), and doing ‘kintu’’, ‘parantu’ and ‘kyo-myo-chyon’ (doubts) about the personality (of God) being Divine or being the Supreme descension, or not accepting even the declarations of the scriptures, then whose ruin will it be? Whose destruction would it be at the end? Your own. How far can we go telling everybody, and believing ourselves, in the wrong or in the false (understanding)? How far can we go? Maybe this lifetime, that's it. And then? And then we're going to rot in suffering, in miseries, in hell, in the cycle of transmigration. It's our destruction because the false philosophy cannot hang around for a long time. It's false! It will finish!
नासतो विद्यते भावो नाभावो विद्यते सतः ।
Nāsato vidyate bhāvo nābhāvo vidyate sataḥ.- Bhagwatam 2.16
“That which is true always remains. It's eternally true. So know such āchāryas (spiritual teachers) to be Me.”
So this is the declaration of the Lord Himself. And listening to such great sages’ talks and declarations, and their wisdom that (originally) comes out of the Lord's mouth, do not just listen and brush it off over there. Listen to it very attentively - shrotavya, and then, mantavya - reflect on everything, and then nididhyāstavya - introspect on every point, and then maitreyi - come to a decision. Make a decision in your life and start treading on the path of love and devotion to God. Only then you'll reap the fruit.
