Who is bhagavan? Anyone who has qualities of aishvarya, virya, yasha, shree, jinanam and vairagya. Who is Purna Bhagavan? Only Krishna or Parthasarathi, Arjuna's charioteer. He is "Krisnastu Bhagavan svayam": God incarnate, Purna Brahmn, Purna Bhagavan.
Arjuna, as per his vow, was to embrace the funeral pyre immediately after sunset if he failed to keep his promise of killing Jayadratha. Suddenly people saw sky darkening; the sun was visible no longer. The sun seemed to have set. Alas, Arjuna would die, honouring his promise.
But it was Krishna who had covered the sun with His Sudarshana Chakra, His special discus, because He willed it so. Jayadratha, who was hiding all day, came out, because now he was safe; Arjuna would have to immolate himself at sunset. Arjuna did his last pranam to Krishna, moving towards the pyre. Krishna said, "Arjuna, you are a Kshatriya, so carry your Gandiva (bow and arrow) even at the time of death". Arjuna did so. When he was to jump, the sun emerged from the clouds.
Krishna asked Arjuna to take aim at Jayadratha. Arjuna was able to keep his promise.
To perform such a miraculous feat by going against the natural order cannot be accomplished by bhagavans. Only Parama Purusha can do this. Dozens of similar stories can be found in the Mahabharata.
Maharshi Garga added:
"Krishna has no parallel". While adoring a baby we say, "How lovely is this child! It is as lovely as the moon". But Krishna's beauty far exceeds that of the moon. About a very intelligent person we say, he is like Veda Vyasa, who compiled the Vedas. By comparison, Krishna was by far the most intelligent personality in existence.
Krishna promised that He would free sinners from sins, so the future of humanity is unmistakably bright. No one should worry about the past. If people constantly think of their Ishta and repeat their Ishta mantra with conceptual understanding, their future is glorious. They will have no cause to lament or cry in sorrow. Only Parama Purusha can state this so emphatically.
Krishna's assertion shows that He is not a god, but God Himself. He is the highest expression of spirituality. If we call Parthasarathi as bhagavan, it will bring Him down to the level of a Mahapurusha. He is Purna Bhagavan, unparalleled. Krishna says in the Gita that all must follow the path shown by Him: "Casting aside all other mental preoccupations accept Me as your last refuge. I will liberate you".
Krishna assures: "I incarnate Myself in this world from age to age for the annihilation of the wicked and the protection of the virtuous". He calls upon people to follow in His footsteps. He gives a clear assurance: "If even wicked persons worship Me with a concentrated mind, I will liberate them from worldly bondages". Krishna says, "If you walk down a road you are bound to get dirty. Those who have abandoned all their previous sins and come to Me, accepting Me as their final refuge, I will free them from the bondage of sin. Their future is glorious". Only Parama Purusha can assure jivas so unequivocally for He is the veritable manifestation of divinity.
"I will take away your sins". No one but Krishna, the Supreme Entity, could say this. No one has ever said this in the past, nor will anyone say it in the future. "I will liberate you from all sins. You have no cause to worry".