There are three divisions of devotees or Kṛṣṇa conscious men. In the third class are those who have no faith. If they are engaged in devotional service officially, for some ulterior purpose, they cannot achieve the highest perfectional stage. Most probably they will slip, after some time. They may become engaged, but because they haven’t complete conviction and faith, it is very difficult for them to continue in devotion or Kṛṣṇa Consciousness. We have practical experience in discharging our missionary activity that some people come and apply themselves to the devotion or Kṛṣṇa consciousness with some hidden motive, and as soon as they are economically a little well-situated, they give up this process and take to their old ways again. It is only by faith that one can advance in Kṛṣṇa consciousness. As far as the development of faith is concerned, one who is well versed in the literatures of devotional service and has attained the stage of firm faith is called a first-class person in Kṛṣṇa consciousness. And in the second class are those who are not very advanced in understanding the devotional scriptures but who automatically have firm faith that Kṛṣṇa bhakti or service to Kṛṣṇa is the best course and so in good faith have taken it up. Thus they are superior to the third class who have neither perfect knowledge of the scriptures nor good faith but by association and simplicity are trying to follow. The third-class person in Kṛṣṇa consciousness may fall down, but when one is in the second class or first class, he does not fall down. Faith in Kṛṣṇa is also divided into three stages and described in Śrīmad-Bhāgavatam. First-class attachment, second-class attachment, and third-class attachment are also explained in Śrīmad-Bhāgavatam in the Eleventh Canto. Those who have no faith even after hearing about Kṛṣṇa and the excellence of devotional service, who think that it is simply eulogy, find the path very difficult, even if they are supposedly engaged in devotional service. For them there is very little hope in gaining perfection. Thus faith is very important in the discharge of devotional service.
Ref >> Srila Prabhupada Vani.