text: which is the fastest declining religion in the world? i heard that it is either christianity or judaism, and also might be hinduism. i saw that europe's population is going to decrease by 20 million in the next decade! so that might reduce christianity.... christianity is the fastest decling religion.  u are right, europe's population is declining because every woman is giving birth to 1.3 kids in average.  and there population is decreasing.    secondly, christians are converting into atheism, agnostic, and islam.  if population is the main cause for the decline of christianity in europe, then islam and athiesm is the cause for its decline in north america.  canada for example was 90% christian, and now recent study say that 60-70% are christians.  in toronto there is a mosque in which christians convert to islam daily.