Urdu has an Islamic identity in India and the neighboring countries even though Muslims speak many languages other than Urdu. How did this association happen.? In Tribune, Prof Tariq Rahman writes that the language went through a change from 1750 CE onwards in which the Indic elements were purged by Muslim poets who then used the language as an identity marker for the religion.
Among the changes which occurred were: the removal of local (bhaka) and Sanskritic words, the substitution of Iranian and Islamic cultural allusions and metaphors in place of Indian and Hindu ones, and the replacement of the Indian conventions about the expression of love (woman to man) by Persian ones (man to woman or adolescent boy). Among the more than 4,000 words purged out were nain (eye), prem (love), mohan (dear one) etc. They do exist in songs and some other forms of poetry, of course, but they were banished from the ghazal. The grounds given in the writings of the poets who did all this — such as Shah Hatim (1699-1786), Imam Baksh Nasikh (d. 1838), Insha Ullah Khan Insha (1752-1818), etc — are not communal. They said that certain words are obsolete, unfashionable and rough. However, the end result was that words of Indic origin were the ones which were purged. That is one reason why I call this movement ‘Islamisation’[How Urdu got associated with Muslims in India? — I]
In Part 2, he looks at the use of Urdu in education, religious debate and printing which helped associate it with Muslims.
As Muslim political power shrank and anxiety spread about why this had happened, the ulema began a movement of education and purification. This they did by writing small books (chapbooks) in the local languages. Thus there are nur namas, wafat namas, jang namas, lahad namas etc. in almost all languages used by Muslims in South Asia and, as it happens, most of them are in Urdu. This movement started in the 18th century and accelerated in the 19th and the 20th centuries. Indeed, if one consults the British reports on printing, one finds that two themes always predominate: religion and love. In some years, one may exceed the other but, as books on history and morals also have a religious colour, it may be true to say that religion mostly predominates printing.
This was a tremendous social change for all religious communities in India. Thus, although there was a secularising trend introduced by the British also, there were more religious texts available in print than ever before. Hence, the consciousness of religious identity grew among all religious communities in India. And within Islam, the consciousness of sectarian identity grew also. Thus, on the one hand the modernist secular classes grew alienated from the religious masses. But on the other, the religious classes also grew alienated from each other and from other religious communities.[How Urdu got associated with Muslims in India — Part II (via Yashwant)]