SB 3.25.22

mayy ananyena bhāvena
 bhaktiṁ kurvanti ye dṛḍhām
mat-kṛte tyakta-karmāṇas
mayi — unto Me; ananyena bhāvena — with undeviated mind; bhaktim — devotional service; kurvanti — perform; ye — those who; dṛḍhām — staunch; mat-kṛte — for My sake; tyakta — renounced; karmāṇaḥ — activities; tyakta — renounced; sva-jana — family relationships; bāndhavāḥ — friendly acquaintances.
Such a sādhu engages in staunch devotional service to the Lord without deviation. For the sake of the Lord he renounces all other connections, such as family relationships and friendly acquaintances within the world.

A person in the renounced order of life, a sannyāsī, is also called a sādhu because he renounces everything — his home, his comfort, his friends, his relatives, and his duties to friends and family. He renounces everything for the sake of the Supreme Personality of Godhead. A sannyāsī is generally in the renounced order of life, but his renunciation will be successful only when his energy is employed in the service of the Lord with great austerity. It is said here, therefore, bhaktiṁ kurvanti ye dṛḍhām. A person who seriously engages in the service of the Lord and is in the renounced order of life is a sādhu. A sādhu is one who has given up all responsibility to society, family, and worldly humanitarianism, simply for the service of the Lord. As soon as he takes his birth in the world, a person has so many responsibilities and obligations — to the public, to the demigods, to the great sages, to the general living beings, to his parents, to the family forefathers and to many others. When he gives up all such obligations for the sake of the service of the Supreme Lord, he is not punished for such renunciation of obligation. But if for sense gratification a person renounces all such obligations, he is punished by the law of nature.