Krishna does not agree to award devotional service to merely anyone. Krishna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in his devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee. — The Nectar of Devotion, Page 14.
Only a pure devotee, whom we deem to be a vaishnava, can be considered sinless.
Pure devotees, who take transcendental pleasure in hearing and chanting the glories of the Lord, do not care for any kind of liberation; even if they are offered the five liberations, they refuse to accept them
Unless one is a completely pure devotee, one cannot enjoy the transcendental bliss derived from chanting and dancing in the ecstasy of the sankirtana movement. Such ecstasy is not possible for an ordinary devotee.
Purport on Srimad Bhagavatam 8.3.20-21
A pure devotee has no demand from the Lord. His only concern is to serve the Lord sincerely and seriously and he is not at all concerned about what will happen in the future.
A preacher has to act in a way that other people don't misunderstand. A pure devotee acts only for the pleasure of Krishna without caring for what others think.
Lecture on "Understanding Krishna through Bhagavad Gita" – Salem – 8th Nov'06