Śuddha-bhakti, the activity of the soul proper—in other words, engagement in the transcendental loving service of the Lord—is performed in a liberated condition. In Bhagavad-gītā (14.26) it is stated:
māṁ ca yo 'vyabhicāreṇa
bhakti-yogena sevate
sa guṇān samatītyaitān
brahma-bhūyāya kalpate
"One who engages in full devotional service, who does not fall down in any circumstance, at once transcends the modes of material nature and thus comes to the level of Brahman."

Avyabhicāriṇī bhakti means unalloyed devotion. A person engaged in devotional service must be free from material motives. In this Kṛṣṇa consciousness movement, one's consciousness must be changed. If consciousness is aimed toward material enjoyment, it is material consciousness, and if it is aimed toward serving Kṛṣṇa, it is Kṛṣṇa consciousness.
(Netar of Instruction).

Just like Bilvamaṅgala Ṭhākura. I think I have several times spoken about Bilvamaṅgala Ṭhākura. So Bilvamaṅgala Ṭhākura, he raised himself in his previous life to the bhāva-bhakti. Bhāva-bhakti means always feeling for Kṛṣṇa. That is the prior stage of loving Kṛṣṇa, bhāva-bhakti. So he, in his previous life he was raised, but somehow or other, he fell down. But it is assured, śucīnāṁ śrīmatāṁ gehe yoga-bhraṣṭo 'bhijāyate: [Bg. 6.41] "Even such Kṛṣṇa consciousness person falls down, he is given chance to take birth in nice brāhmaṇa family or śuci, pure family, and rich family." So Bilvamaṅgala Ṭhākura was very rich man. He was born in a very rich brāhmaṇa family. And he was addicted to prostitute. By some way or other, he had some inclination like that. So he could not finish Kṛṣṇa consciousness. He fallen down. But at a opportune moment he got again the Kṛṣṇa consciousness. How is that? The incidence is that he was performing the śrāddha ceremony of his father, but he had a girlfriend, prostitute. So he wanted to go there very quickly. So somehow or other he finished. And he wanted to go there. But on the road he found that it was very heavily raining and he had to cross the river. So he did everything. And he... The prostitute was... She thought that "Bilvamaṅgala is not coming today. It is very..., so much raining." So when she saw that Bilvamaṅgala is at the door, she was astonished. She said, "Bilvamaṅgala, how did you come here in this rainy, torrents of rainy...?" So he disclosed everything that how he catched one dead body in the river, then he crossed the river, then he jumped over the wall. So she was astonished, and she simply said, "Oh, this much affection if you would have with Kṛṣṇa, how you would have been... Your life would have been nice." Immediately it was... "Oh, Kṛṣṇa...?" Immediately, he left everything. Immediately he left everything and went to Vṛndāvana. He is... So Kṛṣṇa is so nice. Just at the right point He will remind. Yatate ca tataḥ. Here, in the Bhagavad-gītā, yatate ca tato bhūyaḥ saṁsiddhau kuru-nandana, pūrvābhyāsena. He was accustomed, so immediately reminded, immediately.
(Srimad Bhagavatam lecture--1:5:18--New Vrindavana--22nd. June, 1969).

Parā-bhakti. Na aparā. Aparā means material, and parā means transcendental. Parā-bhakti. Without coming to the stage of parā-bhakti, one cannot understand Kṛṣṇa. So that parā-bhakti means one must be freed from all sinful activities. Yeṣām anta-gataṁ pāpam. Not ordinary man. Yeṣām anta-gataṁ pāpaṁ janānām, janānāṁ puṇya-karmaṇām. Those who are always engaged in pious activities, they cannot be engaged in sinful activities. By pious activities, constantly being engaged in pious activities... Then what is that pious activities? Śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ puṇya-śravaṇa-kīrtanaḥ [SB 1.2.17]. Simply if you hear and chant Hare Kṛṣṇa mahā-mantra, puṇya-śravaṇa-kīrtanaḥ, you become pious. Even you, you do not understand, if you simply engage. So the method is very simple. You can become pious and you can become free from all sinful activities.
(Lecture on Bhagavad-Gita--18:67-Ahmedabad, 10th. December, 1972).

Therefore some yoga system, jñāna-yoga system, or dhyāna-yoga system, and there is bhakti-yoga system. That bhakti-yoga system is the perfect. And jñāna-yoga system or dhyāna-yoga system, that is partial understanding, Paramātmā feature. Īśvaraḥ sarva-bhūtānāṁ hṛd-deśe 'rjuna tiṣṭhati [Bg. 18.61]. In that way you can understand, you can come to the platform of understanding samaḥ sarveṣu bhūteṣu. But that is not perfection. Still you have to go. Samaḥ sarveṣu bhūteṣu mad-bhaktiṁ labhate parām [Bg. 18.54]. After realizing samaḥ sarveṣu bhūteṣu, this principle, then you will have to enter into the devotional service, parā-bhakti. Parā-bhakti means transcendental. Bhakti means parā. Bhakti does not mean material. Material, it looks like material activities, but it is not material activities. Just like the example can be given just like a man, a boy is flying kite. He is, what is called, the reel? So sometimes he brings down the kite and sometimes he allows the kite to go. There are two kind of playing. So similarly, the one kind of activity means you are becoming free from the resultant action of activities, and one kind of activity is you are becoming entangled. (Lecture on Srimad Bhagavatam--3:25:29--Bombay, 29th. November, 1974).

Prabhupada:So just like one man is suffering from disease. Some way or other, if he comes in the, what is called, convalescent stage? Means freed from the diseased condition. But if he keeps that convalescent stage rightly, he do not, what is called, again...?

Devotee: Relapse.

Prabhupāda: Relapse. If you do not relapse the disease, then you are nice. Similarly, to keep in Kṛṣṇa consciousness means you are not diseased condition, but do not allow the disease to relapse. That means do not deviate from the devotional service. Māṁ ca avyabhicāreṇa bhakti-yogena yaḥ sevate. Avyabhicāriṇi. Don't misuse the devotional service. Then you are all right. Sa guṇān samatītyaitān brahma-bhūyāya kalpate [Bg. 14.26]. Then you keep yourself always in transcendental position, brahma-bhūtaḥ position. And then from brahma-bhūtaḥ position, you can elevate yourself to the parā bhakti, spontaneous. Parā bhakti means spontaneously, spontaneous devotional service. Automatically, you will be loving Kṛṣṇa.

brahma-bhūtaḥ prasannātmā
na śocati na kāṅkṣati
samaḥ sarveṣu bhūteṣu
mad-bhaktiṁ labhate parām
[Bg. 18.54]
The under, after being in the brahma-bhūtaḥ stage, that devotional service is very sound. Parā bhakti.
(The Nectar of Devotion lecture--Vrindavana, 4th. November, 1972).