In the Vayu Purana an acarya is defined as one who knows the import of all Vedic literatures, abides by their rules and regulations, and teaches his disciples to act in the same way
"One should know the ācārya as Myself and never disrespect him in any way. One should not envy him, thinking him an ordinary man, for he is the representative of all the demigods."
This is a verse from Śrīmad-Bhāgavatam (11.17.27) spoken by Lord Kṛṣṇa when He was questioned by Uddhava regarding the four social and spiritual orders of society. The Lord was specifically instructing how a brahmacārī should behave under the care of a spiritual master. A spiritual master is not an enjoyer of facilities offered by his disciples. He is like a parent. Without the attentive service of his parents, a child cannot grow to manhood; similarly, without the care of the spiritual master one cannot rise to the plane of transcendental service.
The spiritual master is also called ācārya, or a transcendental professor of spiritual science. The Manu-saṁhitā (2.140) explains the duties of an ācārya, describing that a bona fide spiritual master accepts charge of disciples, teaches them the Vedic knowledge with all its intricacies, and gives them their second birth. The ceremony performed to initiate a disciple into the study of spiritual science is called upanīti, or the function that brings one nearer to the spiritual master. One who cannot be brought nearer to a spiritual master cannot have a sacred thread, and thus he is indicated to be a śūdra. The sacred thread on the body of a brāhmaṇa, kṣatriya or vaiśya is a symbol of initiation by the spiritual master; it is worth nothing if worn merely to boast of high parentage. The duty of the spiritual master is to initiate a disciple with the sacred thread ceremony, and after this saṁskāra, or purificatory process, the spiritual master actually begins to teach the disciple about the Vedas. A person born a śūdra is not barred from such spiritual initiation, provided he is approved by the spiritual master, who is duly authorized to award a disciple the right to be a brāhmaṇa if he finds him perfectly qualified. In the Vāyu Purāṇa an ācārya is defined as one who knows the import of all Vedic literatures, abides by their rules and regulations, and teaches his disciples to act in the same way.
Only out of His immense compassion does the Personality of Godhead reveal Himself as the spiritual master. Therefore in the dealings of an ācārya there are no activities but those of transcendental loving service to the Lord. He is the Supreme Personality of Servitor Godhead. It is worthwhile to take shelter of such a steady devotee, who is called āśraya-vigraha, or the manifestation or form of the Lord of whom one must take shelter.
If one poses himself as an ācārya but does not have an attitude of servitorship to the Lord, he must be considered an offender, and this offensive attitude disqualifies him from being an ācārya. The bona fide spiritual master always engages in unalloyed devotional service to the Supreme Personality of Godhead. By this test he is known to be a direct manifestation of the Lord and a genuine representative of Śrī Nityānanda Prabhu. Such a spiritual master is known as ācāryadeva. Influenced by an envious temperament and dissatisfied because of an attitude of sense gratification, mundaners criticize a real ācārya. In fact, however, a bona fide ācārya is nondifferent from the Personality of Godhead, and therefore to envy such an ācārya is to envy the Personality of Godhead Himself. This will produce an effect subversive of transcendental realization.
As mentioned previously, a disciple should always respect the spiritual master as a manifestation of Śrī Kṛṣṇa, but at the same time one should always remember that a spiritual master is never authorized to imitate the transcendental pastimes of the Lord. False spiritual masters pose themselves as identical with Śrī Kṛṣṇa in every respect to exploit the sentiments of their disciples, but such impersonalists can only mislead their disciples, for their ultimate aim is to become one with the Lord. This is against the principles of the devotional cult.
The real Vedic philosophy is acintya-bhedābheda-tattva, which establishes everything to be simultaneously one with and different from the Personality of Godhead. Śrīla Raghunātha dāsa Gosvāmī confirms that this is the real position of a bona fide spiritual master and says that one should always think of the spiritual master in terms of his intimate relationship with Mukunda (Śrī Kṛṣṇa). Śrīla Jīva Gosvāmī, in his Bhakti-sandarbha (213), has clearly explained that a pure devotee's observation of the spiritual master and Lord Śiva as being one with the Personality of Godhead exists in terms of their being very dear to the Lord, not identical with Him in all respects. Following in the footsteps of Śrīla Raghunātha dāsa Gosvāmī and Śrīla Jīva Gosvāmī, later ācāryas like Śrīla Viśvanātha Cakravartī Ṭhākura have confirmed the same truths. In his prayers to the spiritual master, Śrīla Viśvanātha Cakravartī Ṭhākura confirms that all the revealed scriptures accept the spiritual master to be identical with the Personality of Godhead because he is a very dear and confidential servant of the Lord. Gauḍīya Vaiṣṇavas therefore worship Śrīla Gurudeva (the spiritual master) in the light of his being the servitor of the Personality of Godhead. In all the ancient literatures of devotional service and in the more recent songs of Śrīla Narottama dāsa Ṭhākura, Śrīla Bhaktivinoda Ṭhākura and other unalloyed Vaiṣṇavas, the spiritual master is always considered either one of the confidential associates of Śrīmatī Rādhārāṇī or a manifested representation of Śrīla Nityānanda Prabhu.