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SB 9.21.18

tat-prasaṅgānubhāvena
 rantidevānuvartinaḥ
abhavan yoginaḥ sarve
 nārāyaṇa-parāyaṇāḥ
Synonyms: 
tat-prasaṅga-anubhāvena — because of associating with King Rantideva (when talking with him about bhakti-yoga); rantideva-anuvartinaḥ — the followers of King Rantideva (that is, his servants, his family members, his friends and others); abhavan — became; yoginaḥ — first-class mystic yogīs, or bhakti-yogīs; sarve — all of them; nārāyaṇa-parāyaṇāḥ — devotees of the Supreme Personality of Godhead, Nārāyaṇa.
Translation: 
All those who followed the principles of King Rantideva were totally favored by his mercy and became pure devotees, attached to the Supreme Personality of Godhead, Nārāyaṇa. Thus they all became the best of yogīs.
Purport: 

The best yogīs or mystics are the devotees, as confirmed by the Lord Himself in Bhagavad-gītā (6.47):

yoginām api sarveṣāṁ
 mad-gatenāntarātmanā
śraddhāvān bhajate yo māṁ
 sa me yuktatamo mataḥ

“Of all yogīs, he who always abides in Me with great faith, worshiping Me in transcendental loving service, is most intimately united with Me in yoga and is the highest of all.” The best yogī is he who constantly thinks of the Supreme Personality of Godhead within the core of the heart. Because Rantideva was the king, the chief executive in the state, all the residents of the state became devotees of the Supreme Personality of Godhead, Nārāyaṇa, by the king’s transcendental association. This is the influence of a pure devotee. If there is one pure devotee, his association can create hundreds and thousands of pure devotees. Śrīla Bhaktivinoda Ṭhākura has said that a Vaiṣṇava is meritorious in proportion to the number of devotees he has created. A Vaiṣṇava becomes superior not simply by jugglery of words but by the number of devotees he has created for the Lord. Here the word rantidevānuvartinaḥ indicates that Rantideva’s officers, friends, relatives and subjects all became first-class Vaiṣṇavas by his association. In other words, Rantideva is confirmed herein to be a first-class devotee, or mahā-bhāgavata. Mahat-sevāṁ dvāram āhur vimukteḥ: one should render service to such mahātmās, for then one will automatically achieve the goal of liberation. Śrīla Narottama dāsa Ṭhākura has also said, chāḍiyā vaiṣṇava-sevā nistāra pāyeche kebā: one cannot be liberated by his own effort, but if one becomes subordinate to a pure Vaiṣṇava, the door to liberation is open.