Bhagavad Gita 9.3
aprapya mam nivartante
TRANSLATIONThose who are not faithful in this devotional service cannot attain Me, O conqueror of enemies. Therefore they return to the path of birth and death in this material world.
PURPORTThe faithless cannot accomplish this process of devotional service; that is the purport of this verse. Faith is created by association with devotees. Unfortunate people, even after hearing all the evidence of Vedic literature from great personalities, still have no faith in God. They are hesitant and cannot stay fixed in the devotional service of the Lord. Thus faith is a most important factor for progress in Krishna consciousness. In the Caitanya Caritamrta it is said that faith is the complete conviction that simply by serving the Supreme Lord, Sri Krishna, one can achieve all perfection. That is called real faith. As stated in Srimad Bhagavatam 4.31.14
yatha taror mula nisecanena
trpyanti tat-skhanda bhujopasakah
pranopaharac ca yathendriyanam
tathaiva sarvarhanam acyute ja
"By giving water to the root of a tree one satisfies its branches, twigs and leaves, and by supplying food to the stomach one satisfies all the senses of the body. Similarly, by engaging in the transcendental service of the Supreme Lord one automatically satisfies all the demigods and all other living entities." Therefore, after reading Bhagavad-gītā one should promptly come to the conclusion of Bhagavad-gītā: one should give up all other engagements and adopt the service of the Supreme Lord, Krsna.
Now, the development of that faith is the process of Krsna consciousness. There are three divisions of Krsna conscious men. In the third class are those who have no faith. Even if they are officially engaged in devotional service, 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 Krsna consciousness. We have practical experience in discharging our missionary activity that some people come and apply themselves to Krsna 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 Krsna 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 Krsna 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 Krsna-bhakti, or service to Krsna, 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 Krsna consciousness may fall down, but when one is in the second class he does not fall down, and for the first-class person in Krsna consciousness there is no chance of falling down. One in the first class will surely make progress and achieve the result at the end. As far as the third-class person in Kṛṣṇa consciousness is concerned, although he has faith in the conviction that devotional service to Kṛṣṇa is very good, he has not yet gained adequate knowledge of Kṛṣnạ through the scriptures like Śrīmad-Bhāgavatam and Bhagavad-gītā.
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 of gaining perfection. Thus faith is very important in the discharge of devotional service.