One must be fully convinced that Kṛṣṇa or the Supreme Personality of Godhead in His localized aspect as Paramātmā is always within, that He is seeing everything and He always knows what one intends to do. One must thus have firm conviction that Kṛṣṇa as Paramātmā will take care of a soul surrendered to Him. "I shall never be alone," one should think. "Even if I live in the darkest regions of a forest I shall be accompanied by Kṛṣṇa, and He will give me all protection." That conviction is called abhayam, fearlessness. This state of mind is necessary for a person in the renounced order of life.
(Bhagavad-Gita------16:1-3------purport).

This bhakti-yoga begins if a person has the firm conviction that simply discharging devotional service to the Lord in full transcendental love can elevate him to the highest perfectional stage of life. Brahmājī believed in this art of bhakti-yoga; he believed in the instruction of the Lord to execute tapa, and he discharged the function with great penance and thus achieved the great success of seeing the Vaikuṇṭhalokas and the Lord also by personal experience. No one can reach the abode of the Supreme Lord by any mechanical means of the mind or machine, but one can reach the abode of the Vaikuṇṭhalokas simply by following the process of bhakti-yoga because the Lord can be realized only through the bhakti-yoga process. Lord Brahmājī was actually sitting on his lotus seat, and from there, by executing the process of bhakti-yoga in great seriousness, he could see the Vaikuṇṭhalokas with all variegatedness as well as the Lord in person and His associates.
(Srimad Bhagavatam------2:9:18------purport).

To give us practical instructions, Lord Śrī Caitanya Mahāprabhu cleansed the temple twice. His second cleansing was more thorough. The idea was to throw away all the stumbling blocks on the path of devotional service. He cleansed the temple with firm conviction, as is evident from His using His own personal garments for cleaning. Śrī Caitanya Mahāprabhu wanted to see personally that the temple was thoroughly cleansed to the standard of clean marble. Clean marble gives a cooling effect. Devotional service means attaining peace from all disturbances caused by material contamination. In other words, it is the process by which the mind is cooled. The mind can be peaceful and thoroughly cleansed when one no longer desires anything but devotional service.
(Sri Caitanya Caritamrta------2:12:135------purport).

Śrīla Bhaktivinoda Ṭhākura has stated that if one has developed faith in Kṛṣṇa consciousness, he is to be considered an eligible candidate for further advancement in Kṛṣṇa consciousness. Those who have faith are divided into three categories—uttama, madhyama and kaniṣṭha (first class, second class and neophyte). A first-class devotee has firm conviction in the revealed scriptures and is expert in arguing according to the śāstras. He is firmly convinced of the science of Kṛṣṇa consciousness. The madhyama-adhikārī, or second-class devotee, has firm conviction in Kṛṣṇa consciousness, but he cannot support his conviction by citing śāstric references. The neophyte devotee does not yet have firm faith. In this way the devotees are typed.
(Sri Caitanya Caritamrta------2:22:71------purport).

Firm conviction that one should execute the order of the Lord is known as faith. One who has faith is firmly convinced that simply by rendering devotional service to Lord Kṛṣṇa all other activities are automatically performed, including ritualistic duties, sacrifices, yoga and the speculative pursuit of knowledge. In fact, devotional service to the Lord includes everything. As stated in Śrīmad-Bhāgavatam (4.31.14):
yathā taror mūla-niṣecanena
tṛpyanti tat-skandha-bhujopaśākhāḥ
prāṇopahārāc ca yathendriyāṇāṁ
tathaiva sarvārhaṇam acyutejyā
"By watering the root of a tree, one automatically nourishes the branches, twigs, leaves and fruits, and by supplying food to the stomach, one satisfies all the senses. Similarly, by rendering devotional service to Kṛṣṇa, one automatically satisfies the requirements for all other forms of worship and all other spiritual processes." One who is faithful and firmly convinced of this is eligible to be elevated as a pure devotee.

(Teachings of Lord Caitanya).

There are three classes of devotees, according to the degree of conviction. The first-class devotee is conversant with all kinds of Vedic literature and at the same time has the firm conviction mentioned above. He can deliver all others from the pangs of material miseries. The second-class devotee is firmly convinced and has strong faith, but he has no power to cite evidence from revealed scriptures. The third-class devotee is one whose faith is not very strong, but by the gradual cultivation of devotional service he can be promoted to the second- or first-class position. It is said in Śrīmad-Bhāgavatam (11.2.45) that the first-class devotee always sees the Supreme Lord as the soul of all living entities. Thus in seeing all living entities, he sees Kṛṣṇa and nothing but Kṛṣṇa. The second-class devotee places his full faith in the Supreme Personality of Godhead, makes friends with pure devotees, shows favor to innocent persons and avoids those who are atheistic or against devotional service. The third-class devotee engages in devotional service according to the directions of the spiritual master, or engages out of family tradition, and worships the Deity of the Lord, but he has not cultivated knowledge of devotional service, and he does not know a devotee from a nondevotee. Such a third-class devotee cannot actually be considered a pure devotee; he is almost in the devotional line, but his position is not very secure.

(Teachings of Lord Caitanya).

This process is summarized by Rūpa Gosvāmī in the Bhakti-rasāmṛta-sindhu (1.4.15–16): "The first thing required is faith. Due to faith a person associates with pure devotees, and by such association he develops devotional service. As devotional service develops, his misgivings diminish. Then he is situated in firm conviction, and from that conviction he develops a taste for devotional service and advances to the stage of attachment for Kṛṣṇa, whereby he follows the regulative principles of devotional service spontaneously. After that point he makes still further progress and attains the state called bhāva, which is permanent. Such love of God becomes deeper and deeper, until it reaches the highest stage of love of Godhead."

(Teachings of Lord Caitanya).