Srila Bhaktisiddhanta Sarasvati Thakura explains that unless one is directly empowered by the causeless mercy of Krsna, one cannot become the spiritual master of the entire world (jagad-guru). One cannot become an acarya simply by mental speculation. The true acarya presents Krsna 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 acarya, the spiritual master of the entire world, must be considered an incarnation of Krsna’s mercy. Indeed, he is personally embracing Krsna. He is therefore the spiritual master of all the varnas (brahmana, ksatriya, vaisya and sudra) and all the asramas (brahmacarya, grhastha, vanaprastha and sannyasa). Since he is understood to be the most advanced devotee, he is called paramahamsa-thakura. Thakura is a title of honor offered to the paramahamsa. Therefore one who acts as an acarya, directly presenting Lord Krsna by spreading His name and fame, is also to be called paramahamsa-thakura.

From Srila Prabhupada’s purport to Cc Antya 7.13