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.
>>> Ref. VedaBase => SB 3.25.21