As stated in Bhagavad-gītā (14.26) :

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.” Elsewhere in Bhagavad-gītā (9.32) the Lord says:

māṁ hi pārtha vyapāśritya ye ’pi syuḥ pāpa-yonayaḥ striyo vaiśyās tathā śūdrās te ’pi yānti parāṁ gatim

“O son of Pṛthā, those who take shelter in Me, though they be of lower birth — women, vaiśyas [merchants], as well as śūdras [workers] — can approach the supreme destination.”

On the strength of these verses from Bhagavad-gītā, it is evident that although Prahlāda Mahārāja was born in a demoniac family and although virtually demoniac blood flowed within his body, he was cleansed of all material bodily contamination because of his exalted position as a devotee. In other words, such impediments on the spiritual path could not stop him from progressing, for he was directly in touch with the Supreme Personality of Godhead. Those who are physically and mentally contaminated by atheism cannot be situated on the transcendental platform, but as soon as one is freed from material contamination he is immediately fit to be situated in devotional service.