Who is a sādhu? If somebody says, "A man with red dress or a great beard or something . . ." There are so many conception of sādhu. But sādhu is described in the Bhagavad-gītā by the Lord Himself, who is sādhu. He said, api cet su-durācāro bhajate mām ananya-bhāk, sādhur eva sa mantavyaḥ (BG 9.30). Sādhu is one who is unflinchingly engaged in loving service of the Lord. He is a sādhu. That is the test, who is a sādhu. Sādhur eva sa mantavyaḥ.
Kṛṣṇa says, api cet su-durācāraḥ. Even though you find in some devotees some bad behavior, not standard, but because he is a devotee, he is constantly engaged in Kṛṣṇa consciousness, therefore he is sādhu. Even though he has got some bad habits due to his past life, it doesn't matter, because this will stop. Because he has taken to Kṛṣṇa consciousness, all nonsense habit will stop. The switch is off. As soon as one comes to Kṛṣṇa, the switch which impelled one to bad habits, that becomes off immediately.
So just like there is heat, heating, heater, electric heater. If you make the switch off, it still remains hot, but gradually the temperature comes down and it becomes cool. Similarly, if you have got faith, and if you try to engage yourself in Kṛṣṇa consciousness with association of devotee, not alone . . . alone is not possible. Therefore we have established this International Society for Krishna Consciousness. Society is required—sādhu-saṅga—just to give opportunity to persons who have got little faith to develop that faith into Kṛṣṇa consciousness.