Question

Why did roman catholic church began keeping an index of prohibited books from mid 16th centuary

Answer 1

With the establishment of printing press by Johannes Gutenberg, knowledge  started  to be easily disseminated. rare books were printed in lots and made available to people. In fact, from then expression of opinions became much easier.  This however,  the Roman Catholic Church saw it to be a potential threat to their hegemony. Thus they resorted to prohibit the printing of any book that opposed their teachings. Bible, that had started to be translated and available to the mass for individual interpretation was prohibited too.Some books that according to the church contained "theological errors" or in reality were scientific works were also prohibited. Some important works to be prohibited were the works of Diderot, Hugo and Voltaire.Pope Paul IV in 1557 prepared a list of these prohibited books, that was also called the Index.