I am amazed that a scholarly discussion is being given a sectarian (Hindu Muslim) and ethnic twist in this thread. Let us remember that language has no religion neither does it have a nationality. It is just a means of interaction between individuals.
French was the preeminent language of Europe during 19th century. Arabic is spoken in nearly a dozen countries. Persian was the prevalent language in most of the central Asian countries until 19th century. Turkish and its dialects are spoken in about half dozen counties including large parts of Iran. English is a universal language spoken by nearly all nationalities with varied religious beliefs.
Similarly Urdu is neither a Muslim language nor a Pakistani language. It is however most widely understood language in the subcontinent even though in various guises.
I fell in love with Urdu while a young student in Lahore. Primarily because my room mate at the New Hostel at the Gov't College was an Urdu Honors student. Since then, I have carried out my own research on the origins of Urdu and come to the following conclusions. I don’t claim these to be truth and nothing but the truth, however IMO it is a close approximation to the truth.
Mother of all languages in Punjab, Sind and North/Central India is Brij Bhasha or Viraj Bhasha spoken in the area around Mathura, Agra and Delhi. Muslim invasion of Sind in the early 8th century give rise to new language which we now know as Sindhi. Seraiki is probably an older Indo Iranian language but Arabic alphabets were adopted quite early. After reading Heer of Waris Shah, it dawned on me that Punjabi was also very closely related to the Brij Bhasha. Other language of the same family is Hindko. Nearly all of these languages are mutually intelligible.
Amir Khusro (d. 1325) describes himself as a Hinduvi speaking Toork and language used in his poetry is a very mildly Turkicized Brij Bhasha. Devangari script was used until this time. Modern Urdu started in the Deccan (Golconda and Bijapore). Sultan Quli Qutub Shah, who established Qutub Shahi Kingdom in the 16th century, was for my money the founder of Urdu as we know it written in Persian/Arabic script. Qutub Shah was followed by Wali Deccani (d.1704) and Siraj Aurangabadi (d. 1763). By this time Urdu had also established itself in the courts of Delhi and Lukhnow and we had the golden age of Urdu poetry with Sauda (d 1780) Dard (d. 1785) Mir Taqi Mir (d. 1810) Inshaa (d. 817) Naasikh (d. 1834 Ghalib (d. 1869) Mir Anis (d. 1874) and finally Daagh (d. 1905).
Punjab came into prominence a bit late with Mohammad Husain Azad (d. 910) and Iqbal (d 1938). However majority of the modern Urdu poets such as Faiz Ahmad Faiz, Hafeez Jallandhery, Hafeez Hoshiar Puri, Sufi Tabbassam, Saahir Ludhianvi etc. were Punjabis. The great Ahmad Faraaz was Hindko speaking. Mantoo and Abdul Majid Saalik were also Punjabis.
Among non Muslim modern Urdu writers are Ameer Chand Bahaar, Bhagwan Das Ejaz, Sohan Rahi, Indra Mohan Kaif, Deepak Qamar, Asha Prabhat, Inder Shabnam, Kamini Devi, Navroz Kotwal (is he a Parsi, I guess so). Poets of Ghazal include PP Srivastava Rind, Pratpal Singh Betab, Ganesh Bihari Tarz, Premi Romani, Khushbir Singh Shaad, Vijay Arun, Ishwar Dutt Anjum, Rajinder Nath Rahbar, Atul Ajnabi, Harendra Giri Shaad, Om Prabhakar, Tilak Raj Paras, AksLucknowi, Ashshaq Kishtwari, Sardar Panchhi, Prem Bhandari, Vishal Khullar, Shailesh Wafa and Preeta Vajpayee amongst others. Let us not forget Pandit Ratan Naath Sarshaar, creater of 'Fassana Azad'
First Urdu book (Bagho Bahaar) ever printed was at Fort William College Press in Calcutta in 1804 under the blessings of Lord Wellesley. This was translation of Persian ‘Qissa Chahar Dervish' in Urdu by Mir Amman Dehlavi.
At the time of partition in 1947, Lahore boasted the largest Urdu bazaar in the subcontinent; largest numbers of Urdu newspapers were also published in Lahore.
Thus my honorable friends; Urdu is not a Muslim language; it is not a north Indian language. Whatever your mother tongue may be, Urdu doesn’t seek to replace it. It is just a beautiful language adopted by Pakistan as national language and by IOK as their official language. Whether you like it or not, it will remain a beautiful language which it no doubt is.