When these four kinds of persons come to the Supreme Lord for devotional service and are completely purified by the association of a pure devotee, they also become pure devotees. As far as the miscreants are concerned, for them devotional service is very difficult because their lives are selfish, irregular and without spiritual goals. But even some of them, by chance, when they come in contact with a pure devotee, also become pure devotees.
Those who are always busy with fruitive activities come to the Lord in material distress and at that time associate with pure devotees and become, in their distress, devotees of the Lord. Those who are simply frustrated also come sometimes to associate with the pure devotees and become inquisitive to know about God. Similarly, when the dry philosophers are frustrated in every field of knowledge, they sometimes want to learn of God, and they come to the Supreme Lord to render devotional service and thus transcend knowledge of the impersonal Brahman and the localized Paramātmā and come to the personal conception of Godhead by the grace of the Supreme Lord or His pure devotee. On the whole, when the distressed, the inquisitive, the seekers of knowledge, and those who are in need of money are free from all material desires, and when they fully understand that material remuneration has nothing to do with spiritual improvement, they become pure devotees. As long as such a purified stage is not attained, devotees in transcendental service to the Lord are tainted with fruitive activities, the search for mundane knowledge, etc. So one has to transcend all this before one can come to the stage of pure devotional service.

(Bhagavad-Gita-------7:16-------purport).

Actually the transcendental pleasure derived in association with the Supreme Person is far greater than the pleasure derived from impersonal Brahman realization, because of the direct meeting with the eternal form of the Lord. Impersonalists do not directly derive the transcendental pleasure of association with the Lord by hearing of His pastimes. As such, the impersonalists cannot derive any relishable transcendental pleasure from the topics of Bhagavad-gītā, in which the Lord is personally talking with Arjuna. The basic principle of their impersonal attitude does not allow them the transcendental pleasure which is relished by a devotee whose basic principle of understanding is the Supreme Person. The impersonalistic commentary on Bhagavad-gītā is therefore disastrous, because without understanding the transcendental pleasure of the Gītā, the impersonalist wants to interpret it in his own way. If an impersonalist can, however, come in contact with a pure devotee, his transcendental position can be changed for greater elevation. Great sages are therefore recommended to worship the form of the Lord in order to achieve that highest transcendental pleasure.

(Nectar of Devotion).

One cannot attain liberation simply by speculative knowledge. Even though one may be able to distinguish between Brahman and matter, one's liberation will be hampered if one is misled into thinking that the living entity is as good as the Supreme Personality of Godhead. Indeed, one falls down again onto the material platform because considering oneself the Supreme Person, the Supreme Absolute Truth, is offensive. When such a person comes in contact with a pure devotee, he can actually become liberated from material bondage and engage in the Lord's service.

(Sri Caitanya Caritamrta------2:22:21-----purport).

The living entity in the material condition wanders through many varieties of life and many varieties of circumstances, but if he comes in contact with a pure devotee and is intelligent enough to take instructions from the pure devotee regarding the process of devotional service, he can understand the Supreme Personality of Godhead, the origin of Brahman and Paramātmā, without difficulty. In this regard, Śrīla Madhvācārya says:
antaryāmī pratyag-ātmā
vyāptaḥ kālo hariḥ smṛtaḥ
prakṛtyā tamasāvṛtatvāt
harer aiśvaryaṁ na jñāyate
The Lord is present as antaryāmī in everyone's heart and is visible in the individual soul covered by a body. Indeed, He is everywhere at every time and every condition, but because He is covered by the curtain of material energy, to an ordinary person there appears to be no God.

(Srimad Bhagavatam-------7:6:20-23-------purport).

There are those who simply desire liberation, those who are liberated already, even while in this material existence, and those who are actually self-realized. There are many persons in this world who desire liberation, and sometimes they engage in devotional service for this purpose. It is corroborated in Śrīmad-Bhāgavatam (1.2.26) that those who actually desire liberation abandon worship of the demigods, and, without envy, concentrate their minds in the worship of Nārāyaṇa, the Supreme Personality of Godhead. When such persons come in contact with a pure devotee, they engage in the devotional service of Kṛṣṇa and abandon the idea of liberation. In the Hari-bhakti-sudhodaya it is stated:
aho mahātman bahu-doṣa-duṣṭo
'py ekena bhāty eṣa bhavo guṇena
sat-saṅgam ākhyena sukhābahena
kṛtādya no yena kṛśā mumukṣā
"O great soul, although there are many flaws within this miserable life, there is yet one glory—the association of pure devotees. Cultivate such association. By it our desire for liberation diminishes."

(Teachings of Lord Caitanya).