When initiating a person or offering the sacred thread, one vibrates the words oṁ tat sat. Similarly, in all kinds of performance of yajña the object is the Supreme, oṁ tat sat.
(Bhagavad-Gita-----17:26-27-----purport).

The institution of varṇa and āśrama prescribes many regulative duties to be observed by its followers. Such duties enjoin that a candidate willing to study the Vedas must approach a bona fide spiritual master and request acceptance as his disciple. The sacred thread is the sign of those who are competent to study the Vedas from the ācārya, or the bona fide spiritual master. Śrī Śukadeva Gosvāmī did not undergo such purificatory ceremonies because he was a liberated soul from his very birth.
(Srimad Bhagavatam-----1:2:2-----purport).

Generally, a man is born as an ordinary being, and by the purificatory processes he is born for the second time. When he sees a new light and seeks direction for spiritual progress, he approaches a spiritual master for instruction in the Vedas. The spiritual master accepts only the sincere inquirer as his disciple and gives him the sacred thread. In this way a man becomes twice-born, or a dvija. After qualifying as a dvija one may study the Vedas, and after becoming well versed in the Vedas one becomes a vipra. A vipra, or a qualified brāhmaṇa, thus realizes the Absolute and makes further progress in spiritual life until he reaches the Vaiṣṇava stage. The Vaiṣṇava stage is the postgraduate status of a brāhmaṇa. A progressive brāhmaṇa must necessarily become a Vaiṣṇava, for a Vaiṣṇava is a self-realized, learned brāhmaṇa.
(Srimad Bhagavatam-----1:2:2-----purport).

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).

Taking birth in the family of a brāhmaṇa is certainly due to pious activities in one's past life. But still a child who is born in a family of a brāhmaṇa depends for his further reformation upon initiation into acceptance of a sacred thread and many other reformatory processes. But a person who chants the holy name of the Lord, even if born in a family of caṇḍālas, dog-eaters, does not need reformation. Simply by chanting Hare Kṛṣṇa, he immediately becomes purified and becomes as good as the most learned brāhmaṇa.
(Srimad Bhagavatam-----3:33:6-----purport).

It is sometimes remarked that by the chanting process one begins to purify himself and can take birth in his next life in a brāhmaṇa family and then be reformed. But at this present moment, even those who are born in the best brāhmaṇa families are not reformed, nor is there any certainty that they are actually born of brāhmaṇa fathers. Formerly the garbhādhāna reformatory system was prevalent, but at the present moment there is no such garbhādhāna, or seed-giving ceremony. Under these circumstances, no one knows if a man is factually born of a brāhmaṇa father. Whether one has acquired the qualification of a brāhmaṇa depends on the judgment of the bona fide spiritual master. He bestows upon the disciple the position of a brāhmaṇa by his own judgment. When one is accepted as a brāhmaṇa in the sacred thread ceremony under the pāñcarātrika system, then he is dvija, twice-born. That is confirmed by Sanātana Gosvāmī: dvijatvaṁ jāyate. By the process of initiation by the spiritual master, a person is accepted as a brāhmaṇa in his purified state of chanting the holy name of the Lord. He then makes further progress to become a qualified Vaiṣṇava, which means that the brahminical qualification is already acquired.
(Srimad Bhagavatam-----3:33:6-----purport).

A first-class intelligent man is called a brāhmaṇa because he knows the Supreme Brahman, the Absolute Truth. According to the Vedic instructions, tad-vijñānārthaṁ sa gurum evābhigacchet: (MU 1.2.12) to know this science, one must approach a bona fide guru, a spiritual master who will initiate the disciple with the sacred thread so that he may understand the Vedic knowledge. Janmanā jāyate śūdraḥ saṁskārād dhi bhaved dvijaḥ. Becoming a brāhmaṇa through the endeavor of a bona fide spiritual master is called saṁskāra. After initiation, one is engaged in study of the śāstra, which teaches the student how to gain release from materialistic life and return home, back to Godhead.
(Srimad Bhagavatam-----6:5:70-----purport).

The blessings of brāhmaṇas who are not envious, disturbed or puffed up with pride and false prestige and who are fully qualified with truthfulness will be useful. Therefore a class of men must be trained as brāhmaṇas from the very beginning. Brahmacārī guru-kule vasan dānto guror hitam (S,B.10:7:1). The word dāntaḥ is very important. Dāntaḥ refers to one who is not envious, disturbing or puffed up with false prestige. With the Kṛṣṇa consciousness movement, we are trying to introduce such brāhmaṇas in society. Brāhmaṇas must ultimately be Vaiṣṇavas, and if one is a Vaiṣṇava, he has already acquired the qualifications of a brāhmaṇa. Brahma-bhūtaḥ prasannātmā (B.G.18:54). The word brahma-bhūta (S.B.4:30:20) refers to becoming a brāhmaṇa, or understanding what is Brahman (brahma jānātīti brāhmaṇaḥ). One who is brahma-bhūta is always happy (prasannātmā). Na śocati na kāṅkṣati: he is never disturbed about material necessities. Samaḥ sarveṣu bhūteṣu: he is ready to bestow blessings upon everyone equally. Mad-bhaktiṁ labhate parām: (B.G.18:54) then he becomes a Vaiṣṇava. In this age, Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura introduced the sacred thread ceremony for his Vaiṣṇava disciples, with the idea that people should understand that when one becomes a Vaiṣṇava he has already acquired the qualifications of a brāhmaṇa. Therefore in the International Society for Krishna Consciousness, those who are twice initiated so as to become brāhmaṇas must bear in mind their great responsibility to be truthful, control the mind and senses, be tolerant, and so on. Then their life will be successful.
(Srimad Bhagavatam-----10:7:13-15-----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).

There are many Vaiṣṇava families in Bengal whose members, although not actually born brāhmaṇas, act as ācāryas by initiating disciples and offering the sacred thread as enjoined in the Vaiṣṇava tantras. For example, in the families of Ṭhākura Raghunandana Ācārya, Ṭhākura Kṛṣṇadāsa, Navanī Hoḍa and Rasikānanda-deva (a disciple of Śyāmānanda Prabhu), the sacred thread ceremony is performed, as it is for the caste Gosvāmīs, and this system has continued for the past three to four hundred years. Accepting disciples born in brāhmaṇa families, they are bona fide spiritual masters who have the facility to worship the śālagrāma-śilā, which is worshiped with the Deity. As of this writing, śālagrāma-śilā worship has not yet been introduced in our Kṛṣṇa consciousness movement, but soon it will be introduced in all our temples as an essential function of arcana-mārga (Deity worship).
(Sri Caitanya Caritamrta-----1:7:45-----purport).

"Sāndīpani Muni, who formerly offered the sacred thread to Kṛṣṇa and Balarāma, later became Keśava Bhāratī." It is he who offered sannyāsa to Śrī Caitanya Mahāprabhu. There is another statement about Keśava Bhāratī from the Gaura-gaṇoddeśa-dīpikā (117): iti kecit prabhāṣante ’krūraḥ keśava-bhāratī. "According to some authoritative opinions, Keśava Bhāratī is an incarnation of Akrūra." Keśava Bhāratī offered the sannyāsa order to Śrī Caitanya Mahāprabhu in the year 1432 śakābda (A.D. 1510) in Katwa. This is stated in the Vaiṣṇava-mañjuṣā, Part Two.
(Sri Caitanya Caritamrta-----1:9:13-15-----purport).

If a person is born in a śūdra family but has all the qualities of a spiritual master, he should be accepted not only as a brāhmaṇa but as a qualified spiritual master also. This is also the instruction of Śrī Caitanya Mahāprabhu. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura therefore introduced the sacred thread ceremony for all Vaiṣṇavas according to the rules and regulations.
Sometimes a Vaiṣṇava who is a bhajanānandī does not take the sāvitra-saṁskāra (sacred thread initiation), but this does not mean that this system should be used for preaching work. There are two kinds of Vaiṣṇavas—bhajanānandī and goṣṭhy-ānandī. A bhajanānandī is not interested in preaching work, but a goṣṭhy-ānandī is interested in spreading Kṛṣṇa consciousness to benefit the people and increase the number of Vaiṣṇavas. A Vaiṣṇava is understood to be above the position of a brāhmaṇa. As a preacher, he should be recognized as a brāhmaṇa; otherwise there may be a misunderstanding of his position as a Vaiṣṇava. However, a Vaiṣṇava brāhmaṇa is not selected on the basis of his birth but according to his qualities. Unfortunately, those who are unintelligent do not know the difference between a brāhmaṇa and a Vaiṣṇava. They are under the impression that unless one is a brāhmaṇa he cannot be a spiritual master.
(Sri Caitanya Caritamrta-----2:8:128-----purport).

"In this age of quarrel and hypocrisy, the only means of deliverance is the chanting of the holy names of the Lord. There is no other way. There is no other way. There is no other way." A person born in a brāhmaṇa family cannot perform Vedic sacrifices until he is properly purified and has attained his sacred thread. However, according to the present verse of Śrī Caitanya-caritāmṛta (quoted from Śrīmad-Bhāgavatam), it is understood that even a lowborn person can immediately perform sacrifices if he sincerely chants and hears the holy name of the Lord. Sometimes envious people ask how Europeans and Americans in this Kṛṣṇa consciousness movement can become brāhmaṇas and perform sacrifices. They do not know that the Europeans and Americans have already been purified by chanting the holy name of the Lord—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. This is the proof. Śvādo ’pi sadyaḥ savanāya kalpate. One may be born in a family of dog-eaters, but he can perform sacrifices simply by chanting the mahā-mantra.
(Sri Caitanya Caritamrta-----2:16:186-----purport).

Those who find fault in the Western Vaiṣṇavas should consider this statement from Śrīmad-Bhāgavatam and the commentary on this verse by Śrīla Jīva Gosvāmī. In this regard, Śrīla Jīva Gosvāmī has stated that to become a brāhmaṇa one has to wait for purification and undergo the sacred thread ceremony, but a chanter of the holy name does not have to wait for the sacred thread ceremony. We do not allow devotees to perform sacrifices until they are properly initiated in the sacred thread ceremony. Yet according to this verse, an offenseless chanter of the holy name is already fit to perform a fire ceremony, even though he is not doubly initiated by the sacred thread ceremony. This is the verdict given by Devahūti, the mother of Lord Kapiladeva, when He was instructing her in pure Sāṅkhya philosophy.
(Sri Caitanya Caritamrta-----2:16:186-----purport).