Śrī Madhvācārya has brought our attention to this quotation from the Nārāyaṇa-saṁhitā:

dvāparīyair janair viṣṇuḥ pañcarātrais tu kevalaiḥ
kalau tu nāma-mātreṇa pūjyate bhagavān hariḥ

“In the Dvāpara-yuga one could satisfy Kṛṣṇa or Viṣṇu only by worshiping opulently according to the pāñcarātrikī system, but in the Age of Kali one can satisfy and worship the Supreme Personality of Godhead Hari simply by chanting His holy name.” Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura explains that unless one is directly empowered by the causeless mercy of Kṛṣṇa, one cannot become the spiritual master of the entire world (jagad-guru). One cannot become an ācārya simply by mental speculation. The true ācārya presents Kṛṣṇa to everyone by preaching the holy name of the Lord throughout the world. Thus the conditioned souls, purified by chanting the holy name, are liberated from the blazing fire of material existence. In this way, spiritual benefit grows increasingly full, like the waxing moon in the sky. The true ācārya, the spiritual master of the entire world, must be considered an incarnation of Kṛṣṇa’s mercy. Indeed, he is personally embracing Kṛṣṇa. He is therefore the spiritual master of all the varṇas (brāhmaṇa, kṣatriya, vaiśya and śūdra) and all the āśramas (brahmacarya, gṛhastha, vānaprastha and sannyāsa). Since he is understood to be the most advanced devotee, he is called paramahaṁsa-ṭhākura. Ṭhākura is a title of honor offered to the paramahaṁsa. Therefore one who acts as an ācārya, directly presenting Lord Kṛṣṇa by spreading His name and fame, is also to be called paramahaṁsa-ṭhākura.