SB 7.8.1

śrī-nārada uvāca
atha daitya-sutāḥ sarve
śrutvā tad-anuvarṇitam
jagṛhur niravadyatvān
naiva gurv-anuśikṣitam

śrī-nāradaḥ uvāca—Śrī Nārada Muni said; atha—thereupon; daitya-sutāḥ—the sons of the demons (the class friends of Prahlāda Mahārāja); sarve—all; śrutvā—hearing; tat—by him (Prahlāda); anuvarṇitam—the statements about devotional life; jagṛhuḥ—accepted; niravadyatvāt—due to the supreme utility of that instruction; na—not; eva—indeed; guru-anuśikṣitam—that which was taught by their teachers.

Nārada Muni continued: All the sons of the demons appreciated the transcendental instructions of Prahlāda Mahārāja and took them very seriously. They rejected the materialistic instructions given by their teachers, Ṣaṇḍa and Amarka.

This is the effect of the preaching of a pure devotee like Prahlāda Mahārāja. If a devotee is qualified, sincere and serious about Kṛṣṇa consciousness and if he follows the instructions of a bona fide spiritual master, as Prahlāda Mahārāja did when preaching the instructions he had received from Nārada Muni, his preaching is effective. As it is said in the Śrīmad-Bhāgavatam (3.25.25):

satāṁ prasaṅgān mama vīrya-saṁvido
bhavanti hṛt-karṇa-rasāyanāḥ kathāḥ

If one tries to understand the discourses given by the sat, or pure devotees, those instructions will be very pleasing to the ear and appealing to the heart. Thus if one is inspired to take to Kṛṣṇa consciousness and if one practices the process in his life, he is surely successful in returning home, back to Godhead. By the grace of Prahlāda Mahārāja, all his class friends, the sons of the demons, became Vaiṣṇavas. They did not like hearing from their so-called teachers Ṣaṇḍa and Amarka, who were interested only in teaching them about diplomacy, politics, economic development and similar topics meant exclusively for sense gratification.