The purificatory activities begin even before the birth of a child, and the seed-giving reformatory process is called Garbhādhāna-saṁskāra. One who has not undergone such Garbhādhāna-saṁskāra, or spiritual family planning, is not accepted as being of an actual twice-born family. The Garbhādhāna-saṁskāra is followed by other purificatory processes, out of which the sacred thread ceremony is one. This is performed at the time of spiritual initiation. After this particular saṁskāra, one is rightly called twice-born. One birth is calculated during the seed-giving saṁskāra, and the second birth is calculated at the time of spiritual initiation. One who has been able to undergo such important saṁskāras can be called a bona fide twice-born.
(SRIMAD BHAGAVATAM-------1:4:25-------PURPORT).

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.
(SRI CAITANYA CARITAMRTA------1:1:46-------PURPORT).

The principal sign of the age of Kali is that lower-caste śūdras, i.e., men without brahminical culture and spiritual initiation, will be dressed like administrators or kings, and the principal business of such non-kṣatriya rulers will be to kill the innocent animals, especially the cows and the bulls, who shall be unprotected by their masters, the bona fide vaiśyas, the mercantile community.
(SRIMAD BHAGAVATAM-------1:17:1-------PURPORT).

It was at the age of 16 or 17 that he traveled to Gayā with a host of his students and there took his spiritual initiation from Īśvara Purī, a Vaiṣṇava sannyāsī and a disciple of the renowned Madhavendra Purī. Upon his return to Nadia, Nimāi Paṇḍita turned religious preacher, and his religious nature became so strongly represented that Advaita Prabhu, Śrīvāsa and others who had before the birth of Caitanya already accepted the Vaiṣṇava faith were astonished at the change of the young man. He was then no more a contending naiyāyika, a wrangling smārta and a criticising rhetorican. He swooned at the name of Kṛṣṇa and behaved as an inspired man under the influence of his religious sentiment. It has been described by Murāri Gupta, an eyewitness, that he shewed his heavenly powers in the house of Śrīvāsa Paṇḍita in the presence of hundreds of his followers, who were mostly well-read scholars. It was at this time that he opened a nocturnal school of kīrtana in the compound of Śrīvāsa Paṇḍita with his sincere followers. There he preached, there he sang, there he danced, and there he expressed all sorts of religious feelings. Nityānanda Prabhu, who was then a preacher of Vaiṣṇavism and who had then completed his travels all over India, joined him by that time. In fact, a host of paṇḍita preachers of Vaiṣṇavism, all sincere at heart, came and joined him from different parts of Bengal. Nadia now became the regular seat of a host of Vaiṣṇava ācāryas whose mission it was to spiritualize mankind with the highest influence of the Vaiṣṇava creed.
(TEACHINGS OF LORD CAITANYA).

Dīkṣā, or spiritual initiation, is explained in the Bhakti-sandarbha (868) by Śrīla Jīva Gosvāmī:
divyaṁ jñānaṁ yato dadyāt
kuryāt pāpasya saṅkṣayam
tasmād dīkṣeti sā proktā
deśikais tattva-kovidaiḥ
"By dīkṣā one gradually becomes disinterested in material enjoyment and gradually becomes interested in spiritual life."

We have seen many practical examples of this, especially in Europe and America. Many students who come to us from rich and respectable families quickly lose all interest in material enjoyment and become very eager to enter into spiritual life. Although they come from very wealthy families, many of them accept living conditions that are not very comfortable. Indeed, for Kṛṣṇa's sake they are prepared to accept any living condition as long as they can live in the temple and associate with the Vaiṣṇavas. When one becomes so disinterested in material enjoyment, he becomes fit for initiation by the spiritual master.
(NECTAR OF INSTRUCTION).

The brāhmaṇas, kṣatriyas and vaiśyas are called twice-born because for these higher classes of men there is one birth by parental conjugation and there is another birth of cultural rejuvenation by spiritual initiation from the bona fide ācārya, or spiritual master. So a kṣatriya is also twice-born like a brāhmaṇa, and his duty is to give protection to the helpless.
(SRIMAD BHAGAVATAM-------1:17:5-------PURPORT).

One should mentally honor the devotee who chants the holy name of Lord Kṛṣṇa, one should offer humble obeisances to the devotee who has undergone spiritual initiation [dīkṣā] and is engaged in worshiping the Deity, and one should associate with and faithfully serve that pure devotee who is advanced in undeviated devotional service and whose heart is completely devoid of the propensity to criticize others. In order to intelligently apply the sixfold loving reciprocations mentioned in the previous verse, one must select proper persons with careful discrimination. Śrīla Rūpa Gosvāmī therefore advises that we should meet with the Vaiṣṇavas in an appropriate way, according to their particular status. In this verse he tells us how to deal with three types of devotees—the kaniṣṭha-adhikārī, madhyama-adhikārī and uttama-adhikārī. The kaniṣṭha-adhikārī is a neophyte who has received the hari-nāma initiation from the spiritual master and is trying to chant the holy name of Kṛṣṇa. One should respect such a person within his mind as a kaniṣṭha-vaiṣṇava. A madhyama-adhikārī has received spiritual initiation from the spiritual master and has been fully engaged by him in the transcendental loving service of the Lord. The madhyama-adhikārī should be considered to be situated midway in devotional service. The uttama-adhikārī, or highest devotee, is one who is very advanced in devotional service. An uttama-adhikārī is not interested in blaspheming others, his heart is completely clean, and he has attained the realized state of unalloyed Kṛṣṇa consciousness. According to Śrīla Rūpa Gosvāmī, the association and service of such a mahā-bhāgavata, or perfect Vaiṣṇava, are most desirable.
(NECTAR OF INSTRUCTION).

The sun globe is the place where the Supreme Lord is worshiped, spiritual initiation is the means of purification for the spirit soul, and rendering devotional service to the Personality of Godhead is the process for eradicating all one's sinful reactions.
(SRIMAD BHAGAVATAM-------12:11:17-------PURPORT).

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