Because the four Kumaras wanted to engage themselves completely in the service of the Lord as brahmacaris, their refusal to obey their father's order was not irreligious
SB Canto 9
Yadu replied: My dear father, you have already achieved old age, although you also were a young man. But I do not welcome your old age and invalidity, for unless one enjoys material happiness, one cannot attain renunciation.
Renunciation of material enjoyment is the ultimate goal of human life. Therefore the varṇāśrama institution is most scientific. It aims at giving one the facility to return home, back to Godhead, which one cannot do without completely renouncing all connections with the material world. Śrī Caitanya Mahāprabhu said, niṣkiñcanasya bhagavad-bhajanonmukhasya: one who wants to go back home, back to Godhead, must be niṣkiñcana, free from all affinity for material enjoyment. Brahmaṇy upaśamāśrayam: unless one is fully renounced, one cannot engage in devotional service or stay in Brahman. Devotional service is rendered on the Brahman platform. Therefore, unless one attains the Brahman platform, or spiritual platform, one cannot engage in devotional service; or, in other words, a person engaged in devotional service is already on the Brahman platform.
- māṁ ca yo 'vyabhicāreṇa
- bhakti-yogena sevate
- sa guṇān samatītyaitān
- brahma-bhūyāya kalpate
"One who engages in full devotional service, who does not fall down in any circumstance, at once transcends the modes of material nature and thus comes to the level of Brahman." (BG 14.26) If one attains devotional service, therefore, he is certainly liberated. Generally, unless one enjoys material happiness, one cannot attain renunciation. Varṇāśrama therefore gives the opportunity for gradual elevation. Yadu, the son of Mahārāja Yayāti, explained that he was unable to give up his youth, for he wanted to use it to attain the renounced order in the future.
Mahārāja Yadu was different from his brothers. As stated in the next verse, turvasuś coditaḥ pitrā druhyuś cānuś ca bhārata/ pratyācakhyur adharmajñāḥ. Mahārāja Yadu's brothers refused to accept their father's proposal because they were not completely aware of dharma. To accept orders that follow religious principles, especially the orders of one's father, is very important. Therefore when the brothers of Mahārāja Yadu refused their father's order, this was certainly irreligious. Mahārāja Yadu's refusal, however, was religious. As stated in the Tenth Canto, yadoś ca dharma-śīlāya: Mahārāja Yadu was completely aware of the principles of religion. The ultimate principle of religion is to engage oneself in devotional service to the Lord. Mahārāja Yadu was very eager to engage himself in the Lord's service, but there was an impediment: during youth the material desire to enjoy the material senses is certainly present, and unless one fully satisfies these lusty desires in youth, there is a chance of one's being disturbed in rendering service to the Lord. We have actually seen that many sannyāsīs who accept sannyāsa prematurely, not having satisfied their material desires, fall down because they are disturbed. Therefore the general process is to go through gṛhastha life and vānaprastha life and finally come to sannyāsa and devote oneself completely to the service of the Lord. Mahārāja Yadu was ready to accept his father's order and exchange youth for old age because he was confident that the youth taken by his father would be returned. But because this exchange would delay his complete engagement in devotional service, he did not want to accept his father's old age, for he was eager to achieve freedom from disturbances. Moreover, among the descendants of Yadu would be Lord Kṛṣṇa. Therefore, because Yadu was eager to see the Lord's appearance in his dynasty as soon as possible, Yadu refused to accept his father's proposal. This was not irreligious, however, because Yadu's purpose was to serve the Lord. Because Yadu was a faithful servant of the Lord, Lord Kṛṣṇa appeared in his dynasty. As confirmed in the prayers of Kuntī, yadoḥ priyasyānvavāye. Yadu was very dear to Kṛṣṇa, who was therefore eager to descend in Yadu's dynasty. In conclusion, Mahārāja Yadu should not be considered adharma jña, ignorant of religious principles, as the next verse designates his brothers. He was like the four Sanakas (catuḥ-sana), who refused the order of their father, Brahmā, for the sake of a better cause. Because the four Kumāras wanted to engage themselves completely in the service of the Lord as brahmacārīs, their refusal to obey their father's order was not irreligious.