"Sādhavaḥ. Sādhu. Sum and substance of sādhu means devotee. Just like in the Bhagavad-gītā, Kṛṣṇa says, api cet sudurācāro bhajate mām ananya-bhāk, sādhur eva sa mantavyaḥ (BG 9.30). 'He's sādhu.' Who? Ananya-bhāk: 'Without any diversion of attention, he's completely engaged in My devotional service. Even if he has got some minor defects, still, because he is completely surrendered and engaged in My service, he's sādhu.' Sādhur eva sa mantavyaḥ.
So this is the test of sādhu. Sādhu means he must be a devotee. Sādhu does not mean having saffron color and long beard and doing all nonsense. No. Sādhu means... first test is that he is unflinching, without any deviation. Api cet sudurā... bhajate mām ananya-bhāk. Ananya-bhāk means he does not know anything except the service of the Lord. That is the qualification of sādhu.
So similarly, in other places also, the definition of sādhu is there. Sādhu means titikṣavaḥ: he's very tolerant because a sādhu has to face so many opposing elements, and sometimes very offensive, but he has to tolerate. Titikṣavaḥ. At the same time, kāruṇikāḥ, very merciful. The same man who is torturing him, torturing him, he is trying to convince him about Kṛṣṇa. That means very merciful. He's not rejecting. Although he's torturing him, but he's trying to convince him, Kṛṣṇa.
Therefore kāruṇikāḥ. And he is not friend of any particular society or man. Suhṛdaḥ sarva-dehinām: he is well-wisher of all living entities. It doesn't matter whether he's Indian, American or black and white. No, no conception. Or even human being or animal. He is kind to everyone; friend, well-wisher of everyone. Ajāta-śatravaḥ. He does not create any enemy. Such... these are the qualification of sādhu."
(Srila Prabhupada Lecture, London, July 18, 1973)