"There may be many great souls—jñānīs, yogīs and bhaktas, or pure devotees, are also called great souls—but they are very rarely found. One can find many great souls amongst yogīs and jñānīs, but a truly great soul, a pure devotee of the Lord, who is fully surrendered to the Lord, is very rarely found (sa mahātmā sudurlabhaḥ, BG 7.19). A devotee's mind is always calm, quiet and desireless because he is always anyābhilāṣitā-śūnyam (CC Madhya 19.167), having no desire other than to serve Kṛṣṇa as His personal servant, friend, father, mother or conjugal lover. Due to his association with Kṛṣṇa, a devotee is always very calm and cool. It is also significant that within that reservoir all the aquatics were also very calm and quiet. Because the disciples of a devotee have taken shelter of a great soul, they become very calm and quiet and are not agitated by the waves of the material world.
This material world is often described as an ocean of nescience. In such an ocean, everything is agitated. The mind of a great devotee is also like an ocean or a very large lake, but there is no agitation. As stated in Bhagavad-gītā (BG 2.41): vyavasāyātmikā buddhir ekeha kuru-nandana. Those who are fixed in the service of the Lord are not agitated by anything. It is also stated in Bhagavad-gītā (BG 6.22): yasmin sthito na duḥkhena guruṇāpi vicālyate. Even if he suffers some reversals in life, a devotee is never agitated. Therefore whoever takes shelter of a great soul or a great devotee becomes pacified. In the Caitanya-caritāmṛta (CC Madhya 19.149) it is stated: kṛṣṇa-bhakta-niṣkāma, ataeva 'śānta.' A devotee of Lord Kṛṣṇa is always peaceful because he has no desire, whereas the yogīs, karmīs and jñānīs have so many desires to fulfill. One may argue that the devotees have desires, for they wish to go home, back to Godhead, but such a desire does not agitate the mind. Although he wishes to go back to Godhead, a devotee is satisfied in any condition of life. Consequently, the word mahan-manaḥ is used in this verse to indicate that the reservoir of water was as calm and quiet as the mind of a great devotee."
(Śrīmad-Bhāgavatam 4.24.20, Purport)