Bg 9.3

asraddadhanah purusa
dharmasyasya paran-tapa
aprapya mam nivartante
mrtyu-samsara-vartmani

Word for word: 
asraddadhanah — those who are faithless; purusah — such persons; dharmasya — toward the process of religion; asya — this; param-tapa — O killer of the enemies; aprapya — without obtaining; mam — Me; nivartante — come back; mrtyu — of death; samsara — in material existence; vartmani — on the path.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Those 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.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The 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 the Srimad-Bhagavatam (4.31.14),

yatha taror mula-nisecanena
trpyanti tat-skandha-bhujopasakhah
pranopaharac ca yathendriyanam
tathaiva sarvarhanam acyutejya

“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-gita one should promptly come to the conclusion of Bhagavad-gita: one should give up all other engagements and adopt the service of the Supreme Lord, Krishna, the Personality of Godhead. If one is convinced of this philosophy of life, that is faith.

Now, the development of that faith is the process of Krishna consciousness. There are three divisions of Krishna 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 Krishna consciousness. We have practical experience in discharging our missionary activity that some people come and apply themselves to Krishna 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 Krishna 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 Krishna 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 Krishna-bhakti, or service to Krishna, 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 Krishna consciousness may fall down, but when one is in the second class he does not fall down, and for the first-class person in Krishna 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 Krishna consciousness is concerned, although he has faith in the conviction that devotional service to Krishna is very good, he has not yet gained adequate knowledge of Krishna through the scriptures like Srimad-Bhagavatam and Bhagavad-gita. Sometimes these third-class persons in Krishna consciousness have some tendency toward karma-yoga and jnana-yoga, and sometimes they are disturbed, but as soon as the infection of karma-yoga or jnana-yoga is vanquished, they become second-class or first-class persons in Krishna consciousness. Faith in Krishna is also divided into three stages and described in Srimad-Bhagavatam. First-class attachment, second-class attachment and third-class attachment are also explained in Srimad-Bhagavatam in the Eleventh Canto. Those who have no faith even after hearing about Krishna 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.