The simple definition of God is that one who is in full six opulences, he's God. That has been analyzed by great saintly persons, including Lord Brahmā, and it has been decided that the all the six opulences can be found in Kṛṣṇa.
Therefore, Brahmā's decree is given there in the Brahma-saṁhitā,
- īśvaraḥ paramaḥ kṛṣṇaḥ
- sac-cid-ānanda-vigrahaḥ
- anādir ādir govindaḥ
- sarva-kāraṇa-kāraṇam
- (Bs. 5.1)
In the Bhagavad-gītā also it is said, aham ādir hi devānāṁ maharṣīṇāṁ ca sarvaśaḥ (BG 10.2). So that Supreme Personality of Kṛṣṇa is giving us knowledge, the ultimate goal of knowledge. And the living entities, their body, field of activities in this thirteenth chapter they have been very thoroughly described. Although Kṛṣṇa said it is summary. Because they cannot be described in full.