Persons who are always planning to do harm to other living entities are not eligible to understand Kṛṣṇa consciousness and cannot enter into the realm of transcendental loving service to the Lord. Also, there are so-called disciples who become submissive to a spiritual master most artificially, with an ulterior motive. They also cannot understand what Kṛṣṇa consciousness or devotional service is. Persons who, due to being initiated by another sect of religious faith, do not find devotional service as the common platform for approaching the Supreme Personality of Godhead, also cannot understand Kṛṣṇa consciousness. We have experience that some students come to join us, but because of being biased in some particular type of faith, they leave our camp and become lost in the wilderness. Actually, Kṛṣṇa consciousness is not a sectarian religious faith; it is a teaching process for understanding the Supreme Lord and our relationship with Him. Anyone can join this movement without prejudice, but unfortunately there are persons who feel differently. It is better, therefore, not to instruct the science of Kṛṣṇa consciousness to such persons.
(Srimad Bhagavatam----3:32:40----purport).
Generally, materialistic persons are after some name, fame and material gain, so if someone takes to Kṛṣṇa consciousness for these reasons, he will never be able to understand this philosophy. Such persons take to religious principles as a social decoration. They admit themselves into some cultural institution for the sake of name only, especially in this age. Such persons also cannot understand the philosophy of Kṛṣṇa consciousness. Even if one is not greedy for material possessions but is too attached to family life, he also cannot understand Kṛṣṇa consciousness. Superficially, such persons are not very greedy for material possessions, but they are too attached to wife, children and family improvement. When a person is not contaminated by the above-mentioned faults yet at the ultimate issue is not interested in the service of the Supreme Personality of Godhead, or if he is a nondevotee, he also cannot understand the philosophy of Kṛṣṇa consciousness.
(Srimad Bhagavatam----3:32:40----purport).