In the Śrīmad-Bhāgavatam (2.4.18) it is stated that even the lowest, who are called caṇḍālas (dog-eaters), can be purified by association with a pure devotee. Therefore devotional service and the guidance of a pure devotee are so strong that there is no discrimination between the lower and higher classes of men; anyone can take to it. The most simple man taking shelter of the pure devotee can be purified by proper guidance.
(Bhagavad-Gita-------9:32-------purport).

The Supreme Personality of Godhead, Kṛṣṇa, and His plenary portions cannot be understood by mental speculation nor by the nondevotees. If anyone wants to understand the Supreme Personality of Godhead, he has to take to pure devotional service under the guidance of a pure devotee. Otherwise, the truth of the Supreme Personality of Godhead will always be hidden.
(Bhagavad-Gita-------18:55-------purport).

There are many persons who envy Kṛṣṇa because He has explained in Bhagavad-gītā that He is the Supreme and that nothing is above Him or equal to Him. There are many persons who are envious of Kṛṣṇa. Such persons should not be told of Bhagavad-gītā, for they cannot understand. There is no possibility of faithless persons' understanding Bhagavad-gītā and Kṛṣṇa. Without understanding Kṛṣṇa from the authority of a pure devotee, one should not try to comment upon Bhagavad-gītā.
(Bhagavad-Gita-------18:67-------purport).

No one will disagree to partake in a function where good singing, dancing and refreshment are administered. Everyone will attend such a function, and everyone is sure to feel individually the transcendental presence of the Lord. This alone will help the attendant associate with the Lord and thereby purify himself in spiritual realization. The only condition for successfully executing such spiritual activities is that they must be conducted under the guidance of a pure devotee who is completely free from all mundane desires, fruitive activities and dry speculations about the nature of the Lord.
(Srimad Bhagavatam------1:5:36-----purport).

Any person engaged in the devotional service of the Lord is recommended to live in one of the above-mentioned three places, namely Dvārakā, Mathurā or Vṛndāvana. Because devotional service in these three places is magnified, those who go there to follow the principles in terms of instructions imparted in the revealed scriptures surely achieve the same result as obtained during the presence of Lord Śrī Kṛṣṇa. His abode and He Himself are identical, and a pure devotee under the guidance of another experienced devoteecan obtain all the results, even at present.
(Srimad Bhagavatam------1:10:27-----purport).

But even though we are not free from sins, the Lord is kind enough to allow us the facility of seeing Him in His arcā-mūrtis in the temple. The Lord is all-powerful, and therefore He is able to accept our service by presentation of His arcā form. No one, therefore, should foolishly think the arcā in the temple to be an idol. Such an arcā-mūrti is not an idol but the Lord Himself, and to the proportion to which one is free from sins, he is able to know the significance of the arcā-mūrti. The guidance of a pure devotee is therefore always required.
(Srimad Bhagavatam------3:1:17-----purport).

There are different types of material opulence, known technically as bhukti, mukti and siddhi. Bhukti refers to being situated in a very good position, like a position with the demigods in the higher planetary systems, where one can enjoy material sense gratification to the greatest extent. Mukti refers to being disgusted with material advancement and thus desiring to become one with the Supreme. Siddhi refers to executing a severe type of meditation, like that of the yogīs, to attain eight kinds of perfection (aṇimā, laghimā, mahimā, etc.). All who desire some material advancement through bhukti, mukti or siddhi are punishable in due course of time, and they return to material activities. Prahlāda Mahārāja rejected them all; he simply wanted to engage as an apprentice under the guidance of a pure devotee.
(Srimad Bhagavatam------7:9:24-----purport).