Introduction
ताप
the Lord. Every living being, out of many, many billions and trillions of living beings, has a particular relationship with the Lord eternally. That is called  svarūpa . By the process of devotional service, one can revive that  svarūpa , and that stage is called  svarūpa-siddhi —perfection of one's constitutional position. So Arjuna was a devotee, and he was in touch with the Supreme Lord in friendship.
How Arjuna accepted this  Bhagavad-gītā  should be noted. His manner of acceptance is given in the Tenth Chapter.
(12)  arjuna uvāca param brahma param dhāma pavitram paramam bhavān puruṣam śāśvatam divyam ādi-devam ajam vibhum   (13)  āhus tvām ṛṣayaḥ sarve devarṣir nāradas tathā asito devalo vyāsaḥ svayam caiva bravīṣi me   (14)  sarvam etad ṛtam manye yan mām vadasi keśava na hi te bhagavan vyaktim vidur devā na dānavāḥ
“Arjuna said: You are the Supreme Brahman, the ultimate, the supreme abode and purifier, the Absolute Truth and the eternal Divine Person. You are the primal God, transcendental and original, and You are the unborn and all-pervading beauty. All the great sages like Nārada, Asita, Devala, and Vyāsa proclaim this of You, and now You Yourself are declaring it to me. O Kṛṣṇa, I totally accept as truth all that You have told me. Neither the gods nor demons, O Lord, know Thy personality.” (Bg. 10.12-14).
After hearing  Bhagavad-gītā  from the Supreme Personality of Godhead, Arjuna accepted Kṛṣṇa as  Paraṁ Brahma , the Supreme Brahman. Every living being is Brahman, but the supreme living being, or the Supreme Personality of Godhead, is the Supreme Brahman.  Paraṁ dhāma  means that He is the supreme rest or abode of everything,  pavitram  means that He is pure, untainted by material contamination,  puruṣam  means that He is the supreme enjoyer,  divyam , transcendental,  ādi-devam , the Supreme Personality of Godhead,  ajam , the unborn, and  vibhum , the greatest, the all-pervading.
Now one may think that because Kṛṣṇa was the friend of Arjuna, Arjuna was telling Him all this by way of flattery, but Arjuna, just to drive out this kind of doubt from the minds of the readers of  Bhagavad-gītā , substantiates these praises in the next verse when he says that Kṛṣṇa