Hazrat Mirza Ghulam Ahmad (as) took up the writing of Barahin-e-Ahmadiyya at a time when Christian missionaries had hugely spread across India under the umbrella of the British Government. Bible societies were being set up all over India, especially in the Punjab . The force and aggressiveness of the missionaries can be gauged from the fact that in a short period of forty years preceding the writing of this book, 2.9 million Indians were converted to Christianity. Hundreds of tracts and books were produced by the Bible societies, not only attacking Islam, but also the noble personage of the Holy Prophet (sa). This was the beginning of an onslaught on Islam at a time when the Muslims of India were in a state of complete demoralization and dejection. It was not strange, as the Muslims had recently lost their empire after what is called the "Sepoy Mutiny of 1857." 
The Christians were not the only ones in the field against Islam. The Aryah Samaj and the Brahmu Samaj, two powerful movements of orthodox Hindu society, were making inroads among Muslims by maligning the fair face of the Holy Prophet (sa) and degrading the teachings of Islam. 
It was against this background that Hazrat Mirza Ghulam Ahmad (as) of Qadian single-handedly embarked on the monumental task of defending Islam. Directed by a Divine revelation, he decided to write a book with convincing logic and containing irrefutable arguments and data in proof of the truth of Islam. The book completely disarmed his opponents and stopped them from venturing into the field against Islam. 

The Book
 
The first part is confined to describing the numerous beauties and excellences of Islam followed by announcements that if similar excellences were found in any other faith, even half of them or one-fourth, or even one-fifth of the excellences that had been mentioned about Islam, he would give a prize of ten thousand rupees (which was equal to the value of his entire property). He then went a step further and said that if a challenger could not produce the excellences and beauties from his own faith, he was invited to refute the arguments which had been produced in the book with the help and support of the Holy Quran. 

(Taken from An Introduction to the hidden Treasures of Islam)