There are many so-called followers of the Vaiṣṇava cult in the line of Caitanya Mahāprabhu who do not scrupulously follow the conclusions of the śāstras, and therefore they are considered to be apa-sampradāya, which means “outside of the sampradāya.” Some of these groups are known as āula, bāula, kartābhajā, neḍā, daraveśa, sāṅi sahajiyā, sakhībhekī, smārta, jata-gosāñi, ativāḍī, cūḍādhārī and gaurāṅga-nāgarī. In order to follow strictly the disciplic succession of Lord Caitanya Mahāprabhu, one should not associate with these apasampradāya communities.
One who is not taught by a bona fide spiritual master cannot understand the Vedic literature. To emphasize this point, Lord Kṛṣṇa, while instructing Arjuna, clearly said that it was because Arjuna was His devotee and confidential friend that he could understand the mystery of the Bhagavad-gītā. It is to be concluded, therefore, that one who wants to understand the mystery of revealed scriptures must approach a bona fide spiritual master, hear from him very submissively and render service to him. Then the import of the scriptures will be revealed. It is stated in the Vedas (Śvetāśvatara Upaniṣad 6.23):
yasya deve parā bhaktir
yathā deve tathā gurau
tasyaite kathitā hy arthāḥ
“The real import of the scriptures is revealed to one who has unflinching faith in both the Supreme Personality of Godhead and the spiritual master.” Śrīla Narottama dāsa Ṭhākura advises, sādhu-śāstra-guru-vākya, hṛdaye kariyā aikya. The meaning of this instruction is that one must consider the instructions of the sādhu, the revealed scriptures and the spiritual master in order to understand the real purpose of spiritual life. Neither a sādhu (saintly person or Vaiṣṇava) nor a bona fide spiritual master says anything that is beyond the scope of the sanction of the revealed scriptures. Thus the statements of the revealed scriptures correspond to those of the bona fide spiritual master and saintly persons. One must therefore act with reference to these three important sources of understanding.
Ref >> Srila Prabhupada Vani.