In the Vedic literature (Gopāla-tāpanī Upaniṣad 1.1) the following statement appears:
"I offer my respectful obeisances unto Kṛṣṇa, who has a transcendental form of bliss, eternity and knowledge. I offer my respect to Him, because understanding Him means understanding the Vedas and He is therefore the supreme spiritual master." Then it is said, kṛṣṇo vai paramaṁ daivatam: "Kṛṣṇa is the Supreme Personality of Godhead." (Gopāla-tāpanī Upaniṣad 1.3) Eko vaśī sarva-gaḥ kṛṣṇa īḍyaḥ: "That one Kṛṣṇa is the Supreme Personality of Godhead, and He is worshipable." Eko 'pi san bahudhā yo 'vabhāti: "Kṛṣṇa is one, but He is manifested in unlimited forms and expanded incarnations." (Gopāla-tāpanī Upaniṣad 1.21)
The Brahma-saṁhitā (5.1) says,
īśvaraḥ paramaḥ kṛṣṇaḥ
anādir ādir govindaḥ
"The Supreme Personality of Godhead is Kṛṣṇa, who has a body of eternity, knowledge and bliss. He has no beginning, for He is the beginning of everything. He is the cause of all causes."
Elsewhere it is said, yatrāvatīrṇaṁ kṛṣṇākhyaṁ paraṁ brahma narākṛti: "The Supreme Absolute Truth is a person, His name is Kṛṣṇa, and He sometimes descends on this earth." Similarly, in the Śrīmad-Bhāgavatam we find a description of all kinds of incarnations of the Supreme Personality of Godhead, and in this list the name of Kṛṣṇa also appears. But then it is said that this Kṛṣṇa is not an incarnation of God but is the original Supreme Personality of Godhead Himself (ete cāṁśa-kalāḥ puṁsaḥ kṛṣṇas tu bhagavān svayam).
Similarly, in Bhagavad-gītā the Lord says, mattaḥ parataraṁ nānyat: "There is nothing superior to My form as the Personality of Godhead Kṛṣṇa." He also says elsewhere in Bhagavad-gītā, aham ādir hi devānām: "I am the origin of all the demigods." And after understanding Bhagavad-gītā from Kṛṣṇa, Arjuna also confirms this in the following words: paraṁ brahma paraṁ dhāma pavitram-paramaṁ bhavān, "I now fully understand that You are the Supreme Personality of Godhead, the Absolute Truth, and that You are the refuge of everything." Therefore the universal form which Kṛṣṇa showed to Arjuna is not the original form of God. The original is the Kṛṣṇa form.