The two-nation theory, a founding principle of the partition of India in 1947, was effectively shattered with the birth of Bangladesh as a nation-state in 1971. The theory that had advanced the idea of religion -- rather than language, ethnicity or geographical frontiers -- as a unifying factor for Muslims in the South Asian subcontinent, and as justification for the creation of the Dominion of Pakistan, was rejected in 1971 with the emergence of Bangladesh. Maulana Azad -- political leader of independent India and prominent Urdu writer of the 20th century -- wrote in his biography, "It is one of the greatest frauds on the people to suggest that religious affinity can unite areas which are geographically, linguistically and culturally different."  
Bangladesh, the newly-formed nation whose lynchpin was the principle of secularism, saw the rise of a national identity based on the Bengali language. The bloody and glorious Liberation War, a result of 'Bengali nationalism' following decades of economic exploitation and political disenfranchisement of East Pakistan by West Pakistan, symbolised the defiance of religious hegemony in then Dominion of Pakistan. 
Post-liberation, however, the resurgence of Islam challenged the secularist fervour of the newborn country. After the assassination of Sheikh Mujibur Rahman in 1975, political Islam was employed by military rulers as a diversionary tactic (to divert attention from economic problems) and as a tool to create legitimacy. The crisis of legitimacy, faced by the nation in the throes of a military dictatorship and diminishing anti-colonial nationalist sentiment, resulted in resorting back to the familiar concept of Islamic identity -- the very idea which Bengalis had risen against in favour of primordial cultural-linguistic identities.  
Over time, Bangladesh saw its history being written and rewritten, and its secularist and religious identities mutated and reshaped by military and democratic regimes alike. The changing circumstances further complicated the study of shifting political identities that we, as Bangladeshis, have come to historically understand as functions of nationalism, secularism and religion. In order to comprehend the evolution of 'Bengali' and/or 'Bangladeshi' nationalist identity, we must take into account the socio-political and cultural context of Bengal dating back to the pre-modern era. 

Syncretic Hindu/Muslim culture 
In the pre-modern era, Bengal was mainly inhabited by indigenous peoples who had their own way of life; they worshipped their own deities and relied on shifting cultivation systems. The Indo-Aryans, who arrived in the region in the early Vedic period (1750-1000 BCE), brought with them religious and cultural traditions (strong societal hierarchical beliefs, Sanskrit literature) and new agricultural techniques. However, after the conquest of Bengal in 1204 AD by Turkic military commander from the Delhi Sultanate -- Muhammad bin Bakhtiyar Khilji -- the people of Bengal then came under the influence of a new kind of religious identity i.e. Islamic identity.  
The process of Islamisation in Bengal continued along with the absorption of variegated local cultures and agrarian expansion. The 'Bengali Muslim' identity evolved into something unique because of religious intermixing, and similarities in cultural and linguistic heritages. The Bengali culture, also heavily influenced by Buddhist, Sufistic, and Tantrik beliefs, developed into one of co-existence, tolerance and religious inclusion. 
Religious identities in Bengal, during the pre-modern era, were not static or defined within clear boundaries. With the coming of the colonial period, however, reformist Hindu (Shudhi, Sangathan) and Islamic (Wahabi, Farzai, Tarika-e-Mohammad) purification movements emerged in order to form conservative Hindu and Muslim identities. The British, under their policy of 'divide and rule', benefited from the strategy of discerning Hindus and Muslims. Together, British interests and religious movements successfully drove a wedge between Hindu-Muslim relations on the basis of religious difference. Under this syncretic system, however, both the Hindu and Muslim elites sought to dissociate themselves from the assimilated indigenous culture because of religious and economic reasons.

Bengali nationalism
The Muslim ruling elites, that went on to become the most vocal supporters of a separate Muslim homeland, represented Bengal in the Muslim League despite the fact that most of them did not speak or understand the Bengali language spoken by a majority of their 'constituency'. The primarily Punjabi bureaucratic oligarchy, threatened by the cultural and linguistic kinship between Hindus and Muslims, attempted to limit the use of Bengali language -- that by now had become the bedrock of 'Bengali identity' -- since Urdu was seen as the sacred symbol of the Dominion of Pakistan. 
The antagonism shown towards the Bengali culture by the policies pursued by the Muslim League fuelled the language driven resistance movement. Bengali resistance, which resulted in the defeat of the Muslim League in the 1954 provincial elections, voiced the demands of the economically and politically deprived middle-classes of East Bengal. By 1971, Bengali nationalism -- rooted in the Bengali ethno-linguistic identity -- spearheaded the mass movement in East Pakistan's struggle for autonomy that eventually led to independent Bangladesh.

Bangladeshi nationalism
After the assassination of Bangabandhu, subsequent Bangladeshi leaders paved the way for a new kind of nationalism that had Islam at its core. With the reign of the military regime under Ziaur Rahman in the late 1970s came the re-emergence of Islam into politics in Bangladesh. The word 'secularism' was deleted from the Constitution, Article 38 (which had previously banned religion based politics) was amended, and 'Bismillah hir Rahmanir Rahim' was introduced at the top of the preamble. Sheikh Mujib's vision of socialism was redesigned to fit the Islamic concept of social justice; the linguistic understructure of 'Bengali' nationalism transformed into the Islam-centric 'Bangladeshi' nationalism. Although this was an attempt to differentiate Bengalis of Bangladesh from those of West Bengal, the primary motive behind this new brand of identity was to divert attention from the country's fragile economy and rally support on an anti-AL platform. 
Fast forward to early 2000s, the BNP-led coalition saw Islamist parties' participation in the formation of a government for the first time. Socio-political structures were further influenced by Islamic values because of the rise of petro-dollars from the Middle East that facilitated the export of stricter interpretations of Islam to Bangladesh, the increase in the number of madrassas, and the wide-scale patronage of mosques. The revival of Islam and use of communalism continued to serve as the means to an end i.e. consolidate political power, and sway public attention from issues such as mass poverty, underdevelopment and unemployment. 

Politics along fault lines of nationalist identities 
The complex and assimilated nature of the identities of 'Bengali' and 'Bangladeshi' nationalism that AL and BNP have come to be associated with respectively must be recognised to have a more nuanced understanding of their role in contemporary politics. If history is any indication, the construction of a rigid ideology-based nationalist identity can be disadvantageous as it helps create fissures within the polity as all identities have some form of exclusionary element. For instance, where do the Chakma stand under the banner of 'Bengali' nationalism? Do non-Muslims have a place within the realm of 'Bangladeshi' nationalism? 
Such nationalistic interpretations of identity, once deconstructed, leave a lot to be desired in terms of socio-political inclusion of ethnic and religious minorities. However, given the historical ethno-linguistic spirit that gave rise to an independent Bangladesh in 1971 and the resurgence of Islamist politics soon afterwards, both ideas of nationalism remain relevant to the ideological rivalry dominating the political spectrum in Bangladesh today.  
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The writer is a graduate of the University of California, Los Angeles.

Source: 
Contesting Identities in Bangladesh: A Study of Secular and Religious Frontiers, Dr. Sanjay K. Bhardwaj.