Hazrat Mirza Ghulam Ahmad (as) had to embark upon this project because the Aryah Samajists of Qadian following in the footsteps of the Christians thought that the best way of defence of their religion was to launch an attack on the Holy Prophet (sa) and present him as a disgraceful man who indulged in unbridled sexual lust, God forbid (معاذ الله). 

Hazrat Mirza Ghulam Ahmad (as) in this book undertook a critical analysis of the doctrines of the Aryah Samaj. This analysis had to be undertaken to silence the Aryah Samajists who thought that they could get away by throwing stones on the Holy Prophet (sa) while they were sitting in glass houses. He chose one of their doctrines called Niyog and exposed its disgusting nature to public view. How this doctrine is extremely indecent and repugnant to man's natural and moral sense is evident from the fact that the Hindus do not openly practise or preach it.  

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