"It is said that a single pure devotee of the Lord can deliver all the fallen souls of the world. Thus one who is actually in the confidence of a pure devotee like Nārada or Śukadeva Gosvāmī and thus is empowered by one's spiritual master, as Nārada was by Brahmājī, can not only deliver himself from the clutches of māyā, or illusion, but can deliver the whole world by his pure and empowered devotional strength. The comparison to the autumnal rain that falls on muddy reservoirs of water is very appropriate. During the rainy season, all the waters of the rivers become muddy, but in the month of July-August, the autumn season, when there is a slight rainfall, the muddy waters of the rivers all over the world become at once clear. By addition of some chemical, a small reservoir of water like that of a metropolitan waterworks tank can be cleared, but by such a tiny effort it is not possible to clear up all the reservoirs of water like the rivers. A powerful pure devotee of the Lord, however, can deliver not only his personal self but also many others in his association.
In other words, the cleansing of the polluted heart by other methods (like the culture of empiric knowledge or mystic gymnastics) can simply cleanse one's own heart, but devotional service to the Lord is so powerful that it can cleanse the hearts of the people in general, by the devotional service of the pure, empowered devotee. A true representative of the Lord like Nārada, Śukadeva Gosvāmī, Lord Caitanya, the six Gosvāmīs and later Śrīla Bhaktivinoda Ṭhākura and Śrīmad Bhaktisiddhānta Sarasvatī Ṭhākura, etc., can deliver all people by their empowered devotional service."
(Śrīmad-Bhāgavatam 2.8.5, Purport)