In plain words
“Not by works, not by offspring, not by wealth; by sacrifice alone some have attained immortality.” Immortality is won only through sacrifice, is it not?
Then Agni and Indra both took on their own forms and appeared before Śibi. With the words “Your tapas has borne its fruit; your fame is everlasting,” they blessed Śibi, taught him in many ways and went back to their own abodes.
Such tests are for grace alone, not for wrath.
In the Mahābhārata too, such an embodiment of Dharma is set out in many ways. That is why the Mahābhārata has also come to bear the holy name of the Fifth Veda. When the Pāṇḍavas were performing the Aśvamedha yajña, a great king named Mayūradhvaja seized the sacrificial horse, and so Arjuna had to march against him. To defeat him in battle and get the horse back safe was not only the Dharma of a king but the Dharma of the yajña as well. But Kṛṣṇa explained that Mayūradhvaja was a devotee of Kṛṣṇa, given to Dharma and steadfast in Satya, and so it would be very hard to fight him and win. “He fills all his works with śraddhā and Bhakti; he keeps the rules of his āśrama well; he has firm faith in the path of upāsana; he is a man of Jñāna; he is free of envy”: when Kṛṣṇa described Mayūradhvaja in this way, hatred and envy rose up in Arjuna.
Kṛṣṇa saw this; and to set him right, and to proclaim Mayūradhvaja's greatness to the whole world, Kṛṣṇa laid a plan. Saying, “We shall find out why he seized the horse,” the two of them laid aside their royal bearing, put on the guise of brāhmaṇas and went into the palace, wishing to take a meal there. The king welcomed them and received them joyfully. He too sat down with them to eat. But just at that moment Kṛṣṇa played out a drama:
“On the way, a tiger seized the son of this man, my friend, and swallowed half his body; however much the two of us wept and begged it to let the boy go alive, the tiger would not listen. It promised, ‘The moment you bring me half the body of Mayūradhvaja, a body of Dharma, and offer it to me as food, I will hand the boy over to you.’ If you agree to that, we will eat this meal here and now,” the two of them said.
Mayūradhvaja gladly agreed to it. Arjuna saw his spirit of sacrifice and was lost in wonder. After the meal the great king had a saw brought, and in the presence of those two he called his wife and son and told them to cut his body. After some time had passed, Kṛṣṇa saw drops of water falling from the king's left eye and said, “You are not giving your body with complete contentment and a full heart; we do not accept this.”