Urdu is commonly known as "Lashkari Zaban" the term by which one can get to the origin of this lovely language. 

The fact that it is the national language of Pakistan and one of the official languages of India, having a lot fewer native speakers than Punjabi or Sindhi for example but is virtually known by everyone in Pakistan since it is a common language for most.

Urdu literature is mostly popular in Pakistan and there is an abundance of it, the first pieces appearing in the 14th century, and poetry reached its peak in the 19th century. The most developed and known form of poetry is the Ghazal.

This language naturally had words of Hindi, Sanskrit, Arabic, Turkish, Persian, and a few other languages. The language kept evolving with time and had a great influence on the Persian language: the official language of the Mughal period. More than 40% of the words in Urdu have Persian origin.

It is written in the Nastaʿlīq script which was developed in Iran and has borrowed a lot of Persian words.

Later on, when the British ruled on the Indian subcontinent for nearly 150 years, Urdu was directly infused with words from English and indirectly from other foreign languages ( as the English language has multiple origins of words).

Well, it is a commonly misunderstood notion that Urdu is a Muslim's religious language! Well, it couldn't be more wrong. If a language has slightly more importance over other languages from an Islamic perspective, then it would be Arabic (Although there is no linguistic & racial superiority or inferiority in Islam, every race and language is dignified in its own way) because almost all texts of religious importance in Islam are originally in Arabic. In fact, one must know that not even 5% of the total Islamic literature(In Arabic) has been translated into the Urdu language.

Although there is no denial of the fact that Urdu has generally been used in the Indian subcontinent for religious and non-religious academic education of Muslims over the past 200 years and this historical importance and role of Urdu is the reason why it is generally associated with Muslims.

So clearly Urdu is not the "most important language" but yes it is one of the most beautiful languages one can speak, it has words of immense sweetness grouped together in melodious ways which are pleasing to hear and polite to utter.