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):

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.