The earliest known religion of Bengalis was probably Jainism. Out of 24 Jain Tirthamkaras 18 of them spent major portion of their lives in this very Bengal. The district name "Bardhaman" was named after 24th Tirthamkara Bardhaman Mahavir. The next major religion of Bengalis found to beBuddhism, followed by Saivaism and Saktaism. Prior to Sena rules in Bengal, Bengal and its adjoining areas were dominated by Buddhism. About the year 994 AD King Adisur, the first of a famous line of Sen Rajas in Eastern Bengal, at last awoke the great mass of the Hindu population to a sense of its own strength and power, and by right of might displaced the Buddhist kings and founded a new Hindu kingdom of his own in Vikrampur. So far had the Hindu religion fallen during the long centuries of Buddhist rule that King Adisur, it is said, found no Brahmin in all his kingdom who could faithfully perform the ceremonies and ritual of his faith. King Adisur, sending out emissaries far and wide, found that the purest form of Brahminism had been preserved in the city of Kannauj.
In North India Brahmins are divided in five categories. They are 1) Saraswat Brahmins,2) Kannauji Brahmins, 3) Maithili Brahmins,4) Gauriya Brahmins and 5) Utkali Brahmins.
King Vira Singh of Kannauj sent five learned Brahmins, among them Bhatta Narayan as their chief. Welcoming them with respect, the king established them in his capital of Rampal, and there they flourished, they and their descendants, restoring their teaching and example the great doctrines of the Hindu faith. As their descendants became numerous, they scattered themselves into different parts of the Rarh or Radha area. Those settling in the North were called Uttar Rarhi, those on the South as Dakshin Rarhi, those on the east as Bangojas.
The immigrants were attended by five servants, who are the reputed ancestors of the Kayastha caste. In Sanskrit this word means "Standing on the Body", whence Kayasthas claim to be Kshatriyas. But the tradition of a servile origin persisted, and they were forbidden to study the sacred writings. An inherited bent for literature has stood them in good stead: they became adepts in Persian, and English.
These Vedic Brahmins were supposed to have nine gunas (favoured attributes), among which was insistence on same rank marriages. The possession of these gunas caused them to be known as Kulins or those of superior rank. In the next century, King Ballal Sen invited the 56 sons of these five Brahmins to his court and gifted them a village each. He also introduced a merit system (the Kulin Pratha) among the Kulins emphasising nine qualities "as the touchstone of sacerdotal purity", to promote vedic principles in the society, leading to a strict and disciplined lifestyle. Simultaneously Ballal Sen also enforced strict rules on family and marriage rules on Brahmins, leading to the birth of Kulin Brahmins. He also made a similar division for the Kayasthas also. In the case of Kshatriyas / Kayasthas, they were subdivided into three main divisions in Bengal - 1) Vangaja Kayasthas(Eastern Bengal). 2) Uttar Rarhi and 3) Dakshin Rarhi Kayasthas (Western Bengal). As with his predecessor, Ballal Sen insisted on reciprocal exchange of daughters as being one of the qualities. Not all Kulins, however, were willing to accept this ruling on reciprocity. Those who setup their own rules were from then on known as Kshatriya (those learned in the Vedas but of lower rank). Kulin men could marry girls from Kshatriya Brahmin families which were further divided into Siddha (perfect),Sadhya (capable of attaining perfection) and Kashta(difficult) sub-groups.