SB 6.14.5

muktānām api siddhānāṁ
 nārāyaṇa-parāyaṇaḥ
su-durlabhaḥ praśāntātmā
 koṭiṣv api mahā-mune
Word for word: 
muktānām — of those who are liberated during this life (who are unattached to the bodily comforts of society, friendship and love); api — even; siddhānām — who are perfect (because they understand the insignificance of bodily comforts); nārāyaṇa-parāyaṇaḥ — a person who has concluded that Nārāyaṇa is the Supreme; su-durlabhaḥ — very rarely found; praśānta — fully pacified; ātmā — whose mind; koṭiṣu* — out of millions and trillions; api — even; mahā-mune — O great sage.
Translation: 
O great sage, among many millions who are liberated and perfect in knowledge of liberation, one may be a devotee of Lord Nārāyaṇa, or Kṛṣṇa. Such devotees, who are fully peaceful, are extremely rare.
Purport: 

Śrīla Viśvanātha Cakravartī Ṭhākura gives the following purport to this verse. Simply desiring mukti, or liberation, is insufficient; one must become factually liberated. When one understands the futility of the materialistic way of life, one becomes advanced in knowledge, and therefore he situates himself in the vānaprastha order, unattached to family, wife and children. One should then further progress to the platform of sannyāsa, the actual renounced order, never to fall again and be afflicted by materialistic life. Even though one desires to be liberated, this does not mean he is liberated. Only rarely is someone liberated. Indeed, although many men take sannyāsa to become liberated, because of their imperfections they again become attached to women, material activities, social welfare work and so on.

Jñānīs, yogīs and karmīs devoid of devotional service are called offenders. Śrī Caitanya Mahāprabhu says, māyāvādī kṛṣṇe aparādhī: one who thinks that everything is māyā instead of thinking that everything is Kṛṣṇa is called an aparādhī, or offender. Although the Māyāvādīs, impersonalists, are offenders at the lotus feet of Kṛṣṇa, they may nonetheless be counted among the siddhas, those who have realized the self. They may be considered nearer to spiritual perfection because at least they have realized what spiritual life is. If such a person becomes nārāyaṇa-parāyaṇa, a devotee of Lord Nārāyaṇa, he is better than a jīvan-mukta, one who is liberated or perfect. This requires higher intelligence.

There are two kinds of jñānīs. One is inclined to devotional service and the other to impersonal realization. Impersonalists generally undergo great endeavor for no tangible benefit, and therefore it is said that they are husking paddy that has no grain (sthūla-tuṣāvaghātinaḥ). The other class of jñānīs, whose jñāna is mixed with bhakti, are also of two kinds — those who are devoted to the so-called false form of the Supreme Personality of Godhead and those who understand the Supreme Personality of Godhead as sac-cid-ānanda-vigraha, the actual spiritual form. The Māyāvādī devotees worship Nārāyaṇa or Viṣṇu with the idea that Viṣṇu has accepted a form of māyā and that the ultimate truth is actually impersonal. The pure devotee, however, never thinks that Viṣṇu has accepted a body of māyā; instead, he knows perfectly well that the original Absolute Truth is the Supreme Person. Such a devotee is actually situated in knowledge. He never merges in the Brahman effulgence. As stated in Śrīmad-Bhāgavatam (10.2.32):

ye ’nye ’ravindākṣa vimukta-māninas
 tvayy asta-bhāvād aviśuddha-buddhayaḥ
āruhya kṛcchreṇa paraṁ padaṁ tataḥ
 patanty adho ’nādṛta-yuṣmad-aṅghrayaḥ

“O Lord, the intelligence of those who think themselves liberated but who have no devotion is impure. Even though they rise to the highest point of liberation by dint of severe penances and austerities, they are sure to fall down again into material existence, for they do not take shelter at Your lotus feet.” Evidence of this same point is also given in Bhagavad-gītā (9.11), wherein the Lord says:

avajānanti māṁ mūḍhā
 mānuṣīṁ tanum āśritam
paraṁ bhāvam ajānanto
 mama bhūta-maheśvaram

“Fools deride Me when I descend in the human form. They do not know My transcendental nature and My supreme dominion over all that be.” When rascals (mūḍhas) see that Kṛṣṇa acts exactly like a human being, they deride the transcendental form of the Lord because they do not know the paraṁ bhāvam, His transcendental form and activities. Such persons are further described in Bhagavad-gītā (9.12) as follows:

moghāśā mogha-karmāṇo
 mogha-jñānā vicetasaḥ
rākṣasīm āsurīṁ caiva
 prakṛtiṁ mohinīṁ śritāḥ

“Those who are thus bewildered are attracted by demoniac and atheistic views. In that deluded condition, their hopes for liberation, their fruitive activities and their culture of knowledge are all defeated.” Such persons do not know that Kṛṣṇa’s body is not material. There is no distinction between Kṛṣṇa’s body and His soul, but because less intelligent men see Kṛṣṇa as a human being, they deride Him. They cannot imagine how a person like Kṛṣṇa could be the origin of everything (govindam ādi-puruṣaṁ tam ahaṁ bhajāmi). Such persons are described as moghāśāḥ, baffled in their hopes. Whatever they desire for the future will be baffled. Even if they apparently engage in devotional service, they are described as moghāśāḥ because they ultimately desire to merge into the Brahman effulgence.

Those who aspire to be elevated to the heavenly planets by devotional service will also be frustrated, because this is not the result of devotional service. However, they are also given a chance to engage in devotional service and be purified. As stated in Śrīmad-Bhāgavatam (1.2.17):

śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ
 puṇya-śravaṇa-kīrtanaḥ
hṛdy antaḥ-stho hy abhadrāṇi
 vidhunoti suhṛt satām

“Śrī Kṛṣṇa, the Personality of Godhead, who is the Paramātmā [Supersoul] in everyone’s heart and the benefactor of the truthful devotee, cleanses desire for material enjoyment from the heart of the devotee who relishes His messages, which are in themselves virtuous when properly heard and chanted.”

Unless the dirt within the core of one’s heart is cleansed away, one cannot become a pure devotee. Therefore the word su-durlabhaḥ (“very rarely found”) is used in this verse. Not only among hundreds and thousands, but among millions of perfectly liberated souls, a pure devotee is hardly ever found. Therefore the words koṭiṣv api are used herein. Śrīla Madhvācārya gives the following quotations from the Tantra Bhāgavata:

nava-koṭyas tu devānām
 ṛṣayaḥ sapta-koṭayaḥ
nārāyaṇāyanāḥ sarve
 ye kecit tat-parāyaṇāḥ

“There are ninety million demigods and seventy million sages, who are all called nārāyaṇāyana, devotees of Lord Nārāyaṇa. Among them, only a few are called nārāyaṇa-parāyaṇa.

nārāyaṇāyanā devā
 ṛṣy-ādyās tat-parāyaṇāḥ
brahmādyāḥ kecanaiva syuḥ
 siddho yogya-sukhaṁ labhan

The difference between the siddhas and nārāyaṇa-parāyaṇas is that direct devotees are called nārāyaṇa-parāyaṇas whereas those who perform various types of mystic yoga are called siddhas.