It was in the field of culture that the ideas of nationalism was expressed first.
This happened at two levels:
The colonial conquest did not just mean the replacement of one kind of rulers by another. Its effect penetrated deep down to the lives of the ordinary people. In a variety of ways, through the efforts of British rulers and their agents, the culture of then colonial rulers began to spread among the Indian people.
This spread of colonial culture and language produced two responses among the Indian elites. Some of them began to compare the traditional Indian society and culture with the one that existed in Modern England. They questioned some of the elements of the Indian culture. For instance social reformers like Raja Rammohan Roy and Ishwarachandra Vidyasagar worked hard for the eradication of some of the social evils that were a part of the Indian society.
In particular Rammohan Roy attacked the practice of Sati (burning of the widow along with the husband on his death) and Vidyasagar advocated remarriage of widows. Leaders like Jotiba Phule initiated anti-caste movements in Maharashtra. They also made an appeal to the colonial rulers to intervene in the Indian society and bring about reforms, although they did not believe that the European culture was superior to Indian culture. They believe that the British rule represented a modernizing force which could help in the development of the Indian society along modern and rational lines.
At another level, however, the Indian leaders tried to 'defend' and protect Indian culture against what they thought was an encroachment of the colonial culture into the lives of the Indian people. When attempts were made in the 1850s to impose a European dress and other practices on the Indian people, it was resisted by them.
This was also true of those social reformers who admired the British rule and hoped that the colonial rule would, through legislation and other means, introduce modernity in India. Thus Keshub Chandra Sen, a prominent 19th century reformer and a leader of the Brahmo Samaj (formed by Rammohan Roy in 1828) did not like to wear English dress or eat English food. Similarly Ishwarchandra Vidyasagar refused to go to a function hosted by the Lt. Governor because he was required to wear European dress. In this approach cultural rights and practices of the people were seen as very important and the colonial rule was defied on the ground that it was trying to impinge upon them.
The former approach (of questioning the evils of the traditional Indian culture) may look different from the later approach (of resisting any attempt on the part of the colonial rulers to either appropriate or try to change the local Indian cultures). It may appear that the first approach invited British intervention in the Indian society whereas the second approach opposed it. But, as components of Indian nationalism, both the approaches complemented each other.
The idea of cultural nationalism, as it developed in the 19th century was based on a firm rejection of some of the negative features of the traditional Indian culture by, or its integration into, the culture of the colonial rulers. In other words, the 19th century social reformers wanted the Indian culture to become truly modern; but they did not want it to become totally western. In this sense they were opposed to both the traditional culture but also to the modern colonial culture. This was the essence of cultural nationalism as practised in 19th century India.