"The symptoms of a sadhu are that he is tolerant, merciful and friendly to all living entities. He has no enemies, he is peaceful, he abides by the scriptures, and all his characteristics are sublime."
A sadhu, as described above, is a devotee of the Lord. His concern, therefore, is to enlighten people in devotional service to the Lord. That is his mercy. He knows that without devotional service to the Lord, human life is spoiled. A devotee travels all over the country, from door to door, preaching, "Be Krishna conscious. Be a devotee of Lord Krishna. Don't spoil your life in simply fulfilling your animal propensities. Human life is meant for self-realization, or Krishna consciousness." These are the preachings of a sadhu. He is not satisfied with his own liberation. He always thinks about others. He is the most compassionate personality towards all the fallen souls. One of his qualifications, therefore, is karunika, great mercy to the fallen souls. While engaged in preaching work, he has to meet with so many opposing elements, and therefore the sadhu, or devotee of the Lord, has to be very tolerant. Someone may ill-treat him because the conditioned souls are not prepared to receive the transcendental knowledge of devotional service. They do not like it; that is their disease. The sadhu has the thankless task of impressing upon them the importance of devotional service. Sometimes devotees are personally attacked with violence. Lord Jesus Christ was crucified, Haridasa Thakura was caned in twenty-two marketplaces, and Lord Caitanya's principal assistant, Nityananda, was violently attacked by Jagai and Madhai. But still they were tolerant because their mission was to deliver the fallen souls. One of the qualifications of a sadhu is that he is very tolerant and is merciful to all fallen souls. He is merciful because he is the well-wisher of all living entities. He is not only a well-wisher of human society, but a well-wisher of animal society as well. It is said here, sarva-dehinam, which indicates all living entities who have accepted material bodies. Not only does the human being have a material body, but other living entities, such as cats and dogs, also have material bodies. The devotee of the Lord is merciful to everyone -- the cats, dogs, trees, etc. He treats all living entities in such a way that they can ultimately get salvation from this material entanglement. Sivananda Sena, one of the disciples of Lord Caitanya, gave liberation to a dog by treating the dog transcendentally. There are many instances where a dog got salvation by association with a sadhu, because a sadhu engages in the highest philanthropic activities for the benediction of all living entities. Yet although a sadhu is not inimical towards anyone, the world is so ungrateful that even a sadhu has many enemies.
What is the difference between an enemy and a friend? It is a difference in behavior. A sadhu behaves with all conditioned souls for their ultimate relief from material entanglement. Therefore, no one can be more friendly than a sadhu in relieving a conditioned soul. A sadhu is calm, and he quietly and peacefully follows the principles of scripture. A sadhu means one who follows the principles of scripture and at the same time is a devotee of the Lord. One who actually follows the principles of scripture must be a devotee of God because all the sastras instruct us to obey the orders of the Personality of Godhead. Sadhu, therefore, means a follower of the scriptural injunctions and a devotee of the Lord. All these characteristics are prominent in a devotee. A devotee develops all the good qualities of the demigods, whereas a nondevotee, even though academically qualified, has no actual good qualifications or good characteristics according to the standard of transcendental realization.
(Srimad-Bhagavatam 3.25.21, Purport)