Everyone is eligible to become Kṛṣṇa's devotee. One simply has to be trained according to the approved process. It is the work of Kṛṣṇa's confidential devotees to turn everyone into a Kṛṣṇa bhakta. If the confidential devotees do not take up the task of elevating everyone to Kṛṣṇa consciousness, then who will do it? Those who claim to be devotees but do not engage in Kṛṣṇa's service to elevate all living creatures to Kṛṣṇa consciousness are to be considered kaniṣṭha-adhikārīs (people in the lowest stage of devotional service). When one rises to the second platform of devotional service, his business is to propagate Kṛṣṇa consciousness all over the world. Those who are active in the Kṛṣṇa consciousness movement should not remain in the neophyte stage but should rise to the platform of preachers, the second platform of devotional service. Devotional service is so enchanting that even the first-class devotees (uttama-adhikārīs) also come down to the second platform to preach and render service to the Lord for the benefit of the whole world.
(Sri Caitanya Caritamrta----2:24:205----purport).

Above the madhyama-adhikārī is the uttama-adhikārī, who sees everything in relation to the Supreme Lord. Such a devotee does not discriminate between an atheist and a theist but sees everyone as part and parcel of God. He knows that there is no essential difference between a vastly learned brāhmaṇa and a dog in the street, because both of them are part and parcel of the Lord, although they are encaged in different bodies on account of the different qualities of their activities in their previous lives. He sees that the brāhmaṇa particle of the Supreme Lord has not misused his little independence given him by the Lord and that the dog particle has misused his independence and is therefore being punished by the laws of nature by being encaged in the form of a dog. Not considering the respective actions of the brāhmaṇa and the dog, the uttama-adhikārī tries to do good to both. Such a learned devotee is not misled by material bodies but is attracted by the spiritual spark within them.
(Sri Ishopanishad).

Those who imitate an uttama-adhikārī by flaunting a sense of oneness or fellowship but who behave on the bodily platform are actually false philanthropists. The conception of universal brotherhood must be learned from an uttama-adhikārī and not from a foolish person who does not properly understand the individual soul or the Supreme Lord's Supersoul expansion, who dwells everywhere.
It is clearly mentioned in this sixth mantra that one should "observe," or systematically see. This means that one must follow the previous ācāryas, the perfected teachers. Anupaśyati is the exact Sanskrit word used in this connection. Anu means "to follow," and paśyati means "to observe." Thus the word anupaśyati means that one should not see things as he does with the naked eye but should follow the previous ācāryas. Due to material defects, the naked eye cannot see anything properly.
(Sri Ishopanishad).

According to the Bhagavad-gītā (18.54), only one who is already on the liberated platform (brahma-bhūta (S.B.4:30:20)) can become an uttama-adhikārīdevotee and see every living being as his own brother. This vision cannot be had by politicians, who are always after some material gain. One who imitates the symptoms of an uttama-adhikārī may serve another's outward body for the purpose of fame or material reward, but he does not serve the spirit soul. Such an imitator can have no information of the spiritual world. The uttama-adhikārī sees the spirit soul within the material body and serves him as spirit. Thus the material aspect is automatically served.
(Sri Ishopanishad).

Except for the madhyama-adhikārī and uttama-adhikārī discussed above, no one can correctly see the spiritual position of a living being. The living entities are qualitatively one with the Supreme Lord, just as the sparks of a fire are qualitatively one with the fire. Yet sparks are not fire as far as quantity is concerned, for the quantity of heat and light present in the sparks is not equal to that in fire. The mahā-bhāgavata, the great devotee, sees oneness in the sense that he sees everything as the energy of the Supreme Lord. Since there is no difference between the energy and the energetic, there is the sense of oneness. Although from the analytical point of view heat and light are different from fire, there is no meaning to the word "fire" without heat and light. In synthesis, therefore, heat, light and fire are the same.
(Sri Ishopanishad).

By the process of executing regulated devotional service, one is actually elevated onto the transcendental stage, beyond the material modes of nature. At that time one's heart becomes illuminated like the sun. The sun is far above the planetary systems, and there is no possibility of its being covered by any kind of cloud; similarly, when a devotee is purified like the sun, from his pure heart there is a diffusion of ecstatic love which is more glorious than the sunshine. Only at that time is the attachment to Kṛṣṇa perfect. Spontaneously, the devotee becomes eager to serve the Lord in his ecstatic love. At this stage the devotee is on the platform of uttama-adhikārī, perfect devotion. Such a devotee has no agitation from material affections and is interested only in the service of Rādhā and Kṛṣṇa.
(Nectar of Devotion).