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In Srila Bhaktivinode Thakur's book called Jaiva Dharma. Pg. 384 Chapter 24 translated by Sarva bhavana das…there is the following text….it’s a question-answer section between two great souls.

Vijaya, Gurudeva, I now understand that the mayavadis, karmis jnanis and yogis in general are all namaparadhis. Should a suddha bhakta participate in harinama sankirtan with them?

Raghunatha das babaji – "Vaishnavas are advised not to join an assembly of chanters that comprises a majority of namaparadhis, and also if the lead singer is a namaparadhi.

However in a kirtana group consisting mainly of suddha bhaktas and bhaktas upon the platform of namabhasa there is all good reason to participate. And such participation will bring great satisfaction and spiritual bliss.

And here is what Srila Prabhupada says on this same idea…….

(this file composed by Yashoda Nandana prabhu)

Who and who not to associate with

We should befriend and offer special respect to persons who are developed in Krishna consciousness. Other living entities are undoubtedly part and parcel of the Supreme Lord, but because their consciousness is still covered and not developed in Krishna consciousness, we should renounce their association. It is said by Vishvanatha Chakravarti Thakura that even if one is a Vaishnava, if he is not of good character his company should be avoided, although he may be offered the respect of a Vaishnava. Anyone who accepts Vishnu as the Supreme Personality of Godhead is accepted as a Vaishnava, but a Vaishnava is expected to develop all the good qualities of the demigods.

The exact meaning of the word sattvena is given by Sridhara Swami as being synonymous with dhairyena, or patience. One must perform devotional service with great patience. One should not give up the execution of devotional service because one or two attempts have not been successful. One must continue. Sri Rupa Gosvami also confirms that one should be very enthusiastic and execute devotional service with patience and confidence. Patience is necessary for developing the confidence that “Krishna will certainly accept me because I am engaging in devotional service.” One has only to execute service according to the rules and regulations to insure success.

Srimad-Bhagavatam: Canto 3: "The Status Quo”: SB 3.29: Explanation of Devotional Service by Lord Kapila: SB 3.29.16: PURPORT

Srila Vishvanatha Chakravarti Thakura advises that a Vaishnava who is not of good character should be avoided. A Vaishnava is one who has taken the Supreme Personality of Godhead as the ultimate goal of life, but if one is not pure and still has motives, then he is not a Vaishnava of the first order of good character. One may offer his respects to such a Vaishnava because he has accepted the Supreme Lord as the ultimate goal of life, but one should not keep company with a Vaishnava who is in the mode of ignorance.

Srimad-Bhagavatam: Canto 3: "The Status Quo”: SB 3.29: Explanation of Devotional Service by Lord Kapila: SB 3.29.8: PURPORT

In Bhagavad-gita there is reference to bodhayantah parasparam, “discussing among themselves.” Generally pure devotees utilize their valuable time in chanting and discussing various activities of Lord Krishna or Lord Chaitanya amongst themselves. There are innumerable books, such as the puranas, Mahabharata, Bhagavatam, Bhagavad-gita and Upanishads, which contain countless subjects for discussion among two devotees or more. Friendship should be cemented between persons with mutual interests and understanding. Such persons are said to be sva jati, “of the same caste.”

The devotee should avoid a person whose character is not fixed in the standard understanding; even though he may be a Vaishnava, or a devotee of Krishna, if his character is not correctly representative, then he should be avoided. One should steadily control the senses and the mind and strictly follow the rules and regulations, and he should make friendship with persons of the same standard.

Srimad-Bhagavatam: Canto 3: "The Status Quo”: SB 3.29: Explanation of Devotional Service by Lord Kapila: SB 3.29.17: PURPORT

"… If one thinks that there are many pseudo devotees or nondevotees in the Krishna Consciousness Society, one can keep direct company with the spiritual master, and if there is any doubt, one should consult the spiritual master…"

Sri Chaitanya-charitamrita – 1975 Edition: Cc. Madhya-lila: Madhya 19: Lord Sri Chaitanya Mahaprabhu Instructs Srila Rupa Gosvami: Madhya 19.157: PURPORT

"… It is also said, phalena pariciyate: one’s success or defeat in any activity is understood by its result. There are many karmis in the dress of devotees, but the Supreme Personality of Godhead can detect their purpose. The karmis want to use the property of the Lord for their selfish sense gratification, but a devotee endeavors to use the Lord’s property for God’s service. Therefore a devotee is always distinct from the karmis, although the karmis may dress like devotees. As confirmed in Bhagavad-gita (3.9), yajnarthat karmano ’nyatra loko ’yam karma-bandhanah. One who works for Lord Vishnu is free from this material world, and after giving up his body he goes back home, back to Godhead. A karmi, however, although externally working like a devotee, is entangled in his nondevotional activity, and thus he suffers the tribulations of material existence. Thus from the results achieved by the karmis and devotees, one can understand the presence of the Supreme Personality of Godhead, who acts differently for the karmis and jnanis than for the devotees.

Srimad-Bhagavatam: Canto 8: "Withdrawal of the Cosmic Creations”:

SB 8.9: The Lord Incarnates as Mohini-Murti: SB 8.9.28: PURPORT  

An actual Vaishnava is very pleased to accept another Vaishnava who is bestowing the Lord's mercy. A mundane person in the dress of a Vaishnava should not be respected but rejected. This is enjoined in the sastra (upeksa). The word upeksa means neglect. One should neglect an envious person. A preacher's duty is to love the Supreme Personality of Godhead, make friendships with Vaishnavas, show mercy to the innocent and reject or neglect those who are envious or jealous. There are many jealous people in the dress of Vaishnavas in this Krishna consciousness movement, and they should be completely neglected. There is no need to serve a jealous person who is in the dress of a Vaishnava. When Narottama dasa Thakura says chadiya vaisnava seva nistara payeche keba, he is indicating an actual Vaishnava, not an envious or jealous person in the dress of a Vaishnava.

Sri Chaitanya-charitamrita – 1975 Edition: Cc. Madhya-lila: Madhya 1: The Later Pastimes of Lord Sri Chaitanya Mahaprabhu: Madhya 1.218: PURPORT