Srila Bhaktisiddhanta Saraswati Thakur explains this point as follows:
One who can ascertain the forefathers of his family beginning from Lord Brahma without any break, who can confirm that they have all properly undergone the ten purificatory processes, who can prove that there was never any intercaste marriages in his family and who can establish that before each pregnancy the appropriate samskaras have been performed -he is a seminal brahmana. The Vedic statement: asta varsam brahmanam upanayita, "a brahmana boy should be brought to an acarya for sacred thread ceremony at the age of eight" and the smrti statement: garbhastame bde kurvita brahmanasyopanayanam, "a brahmana should be awarded the sacred at the age of eight" refer only to the above mentioned brahmanas, not to the sons of brahmanas who have been accepted as brahmanas only for a few generations. In other words, it does not refer to those in whose family an intercaste marriage took place, to those who have undergone garbhadhana-samskara only once, or to those who have improperly or imperfectly undergone samskaras. "Brahmanas from a few generations" means either the descendants of those who have taken birth in other varnas yet qualified themselves as brahmanas or those who were qualified brahmanas according to the process mentioned in the Vedic literatures. Just as the Gargya brahmanas were descendants of Sini, the son of Garga, the Maudgalya brahmanas were descendants of Mudgala, the Vasistha brahmanas were descendants of sage Vasistha, the son of Mitra from the womb of Urvasi, and so on innumerable such brahmana families, though not saukra brahmanas, will remain glorified as brahmanas as long as they continue to remain qualified, which is the root foundation of brahmanism.
If the descendants of either seminal or professional brahmanas do not possess the necessary qualifications, then they are also fallen. It is stated in Mahabharata, Vana parva, Chapter 215 as follows:
Dambhiko duskritah prajnah
sudrena sadriso bhavet
"If a brahmana is engaged in sinful activities, if he is proud, or if he is a miscreant, then he falls to the level of a Sudra". According to this statement from the scriptures it is difficult in this age of Kali to ascertain who is in a pure unbroken seminal brahmana line and who is not. Therefore, through the ascending process it is very uncommon to identify a real twice born or thrice born. But those who receive the knowledge of serving the transcendental Lord through the process of disciplic succession and follow the Vedic injunctions, they are becoming qualified to undergo the samskaras mentioned in the Vedic literature. In this age of Kali, there is no other way of becoming purified.
Asuddhah sudra-kalpa hi
suddhir na sotra-vartmana
"The brahmanas born in the age of Kali are merely sudras. Their so- called Vedic path of karma is polluted and cannot purify them. They can only be purified by following the path of the Agamas or Pancaratras". Therefore, at present, a brahmana should accepted as such only if he follows the rules and regulations of the satvata- agamas, tantras, or Pancaratras, since the scriptures clearly mention that brahmanas born in this age of Kali are impure, or their purity is not maintained through seminal descent. Therefore according to Vedic injunctions, they cannot even become twice born in their impure state, and what to speak of becoming thrice born.
According to the process of Vaisnava literatures in pursuance of the Vedas, an impure person can become pure only by spiritual initiation; there cannot be any other arrangement for purification. It dies not matter in which family one is born, either in a so-called brahmana family or a lower family, if he wants to be purified in this age of Kali then he should earn his qualification to be twice born by firs t being initiated according to the rules and regulations of the Vedic Pancaratras and thereafter accept the signs of a twice born. It is stated in the scriptures:
Yatha kancanatam yati
kamsyam rasa vidhanatah
tatha diksa vidhanena
dvijatvam jayate nrinam
"As bell metal is turned to gold when mixed with mercury in an alchemical process, so one who is properly trained and initiated by a bona fine spiritual master immediately becomes a brahmana". Every human being must take Vaisnava initiation according to the proper Vedic Pancaratra process. Then he becomes a qualified twice born who must then accept the appropriate signs such as the sacred thread.
The paramahamsas, who are actually eternally perfect pure souls, are beyond the principles of varnasrama. They need not again purify themselves. Therefore they may also not accept the sacred thread, This does not mean, however, that they are in any way less than a thrice born. They are the spiritual masters of the brahmanas, and the brahmanas are their servants.
In ancient times there was only one Varna called hamsa. Later on, according to qualities and work, the four varnas were created (there were not four varnas in the beginning). Of course, there is a gulf of difference between the four varnas and their respective qualifications. Intelligent people should carefully consider this. Apart from the four varnas created by the Lord, the system of a son s purely inheriting the Varna of is father has also been accepted. This is called seminal Varna. But every reader of the scriptures knows that the seminal process is not the only way of ascertaining one s Varna. He certainly knows that among the one hundred sons of Risabhadeva, eighty-one of them became brahmanas, nine of them became ksatriyas, and nine became Vaisnavas. Apart from brahmana sons such as Saunaka, Gritsamada also had ksatriya, Vaisya and Sudra sons. The sons of the ksatriya Duritaksaya (Trayyaruni, Kavi and Puskararuni) became brahmanas. In the dynasty of King Ajamidha, the brahmana Priyamedha was born. There are hundreds of such examples in the scriptures. Though Sathakopa Dasa, the spiritual master of Sri Sri Ramanujacarya Prabhu, was born in the family of a Sudra, he was a brahmana. And in the Gaudiya Vaisnava society, in the families of Srila Rasikananda Prabhu, Srila Raghunandana, Sri Hari Hoda, and others, performance of the twice born samskara is still current. This spiritual position of brahmanas, attained through spiritual initiation has been accepted by intelligent people from time immemorial. There is no need to be surprised by seeing or hearing this.
(Weekly Gaudiya, Vol. 1, Part 28)