BG Chap 9 - The faithful and the faithless
Those who do not have firm faith in the dharma of devotional service to Kṛṣṇa, though its strength is well known in the scriptures because they think these statements are exaggeration, do not attain the Lord but remain in this world. Even if they perform other practices to attain the Lord, while ignoring bhakti, they do not attain the Lord. They remain fixed on the path of saṁsāra, which brings about repeated birth and death.
Faith is created by association with devotees. Faith is the complete conviction that simply by serving the Supreme Lord, Śrī Kṛṣṇa, one can achieve all perfection. There are three divisions of Kṛṣṇa conscious men. In the third class are those who have no faith. They have neither perfect knowledge of the scriptures nor good faith but by association and simplicity are trying to follow. 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. 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.