Vyasa Imparting Knowledge to Madhva
Sadhu is one who gives quotations from authorized sastra. Nothing is manufactured by him. Whatever he speaks, he'll immediately give evidence from the sastra to support it.
"He's sadhu." Who? Ananya-bhak: "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 sadhu." Sadhur eva sa mantavyah. So this is the test of sadhu. Sadhu means he must be a devotee. Sadhu does not mean having saffron color and long beard and doing all nonsense. No. Sadhu means... First test is that he is unflinching, without any deviation. Api cet sudura..., bhajate mam ananya-bhak. Ananya-bhak means he does not know anything except the service of the Lord. That is the qualification of sadhu. So similarly, in other places also, the definition of sadhu is there. Sadhu means titiksavah: he's very tolerant because a sadhu has to face so many opposing elements and sometimes very offensive, but he has to tolerate. Titiksavah. At the same time, karunikah, very merciful. The same man who is torturing him, torturing him, he is trying to convince him about Krsna. That means very merciful. He's not rejecting. Although he's torturing him, but he's trying to convince him, Krsna. Therefore karunikah. And he is not friend of any particular society or man. Suhrdah sarva-dehinam. 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. Ajata-satravah. He does not create any enemies. ....These are the qualifications of sadhu."
Srila Prabhupada Room Conversation, 07-18-73, London
It is recommended to follow the sadhus, who are the great superior and saintly personalities. Sadhu means one who is obeying the rules and regulation of sastra.
Nobody can become a sadhu if he does not accept the principles of scripture. One has to see a sadhu by corroborating his statements according to the sastras or another sadhu.
The sadhu will speak only on the authority of sastra. Sadhu never changes or malinterprets sastra. He thoroughly understands sastra as it has come down through disciplic succession, from Vyasadeva, Narada, down to many acaryas like Ramanujacarya, Madhvacarya, Visnuswami, and Lord Caitanya. When it has been accepted by sadhu, then it can be accepted as scripture.
Good association with like-minded devotees is required for one interested in spiritual advancement. Such association is called sadhu-sanga, and association with the devotees can change ones consciousness from material to spiritual. That is the purpose of the Krsna consciousness movement.
"This is the definition of sadhu. Saint, who is a saint? A saint is called who is very tolerant, tolerant, tolerant to the utmost. You have got very good example of Lord Jesus Christ, a great saint, or sadhu also. He was, of course, more than sadhu. Now, just see his behavior, how much tolerant he was. He was being crucified and he was praying God, "O Lord, forgive these people, what they are doing." That, this is the significance of sadhu. Titiksavah karunikah. For their personal sake, they're always very tolerant, but they are very kind to all people, all living entities, very kind. In spite of their all disadvantages, they try to give something, real knowledge, to the people in general. Karunikah. And suhrdah sarva-dehinam. And a sadhu is not a friend of a particular class, particular community or particular country. No. A sadhu, a saint, is he who is friend of all, not only of human being, even of animals and less than animals. These are the qualification of sadhu. Ajata-satravah."
Srila Prabhupada Lecture on Bhagavad-gita, 07-20-66, New York
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.