Final answer:
The Catholic Church placed books on the Index of Prohibited Books mainly to prevent the spread of heretical ideas and to maintain its authority during a time when new philosophies and scientific discoveries were emerging.
Step-by-step explanation:
The Catholic Church placed books on the Index of Prohibited Books to control the spread of heretical ideas. In the context of the Renaissance and the rise of humanism, the Church found itself contending with a wave of new philosophies and scientific discoveries that contradicted long-held doctrinal beliefs. Figures like Martin Luther and Galileo, who used the printing press to disseminate their ideas, represented a significant challenge to the Church's authority. To protect its interpretative control over Christianity, and to maintain societal order, the Church censored works it deemed contrary to its teachings.
Books that encouraged skepticism towards religious decrees or those by authors that directly challenged the Church's claims were often prohibited. This was a time when new information and scientific knowledge started to reshape the standard way of thinking, making control over what could be published and read an important power tool for the Church. Thus, the main reason for the Index of Prohibited Books was to prevent the spread of ideas that could disrupt the religious and social order that the Church sought to maintain.