Not every work of a non Catholic was forbidden but only among others every work on religion by a non Catholic I don t know whether the Opus Dei adheres to the index or as it is said censures new books but I may say that if it wouldn t be to blame for it It has every right over its membership and followers until forbidden so by the Holy See that the Holy See itself over all the Church had when it did publish the index Though this has been officially denied I believe the action of H Em Bertone against the Da Vinci Code was not far from censuring and a quite just censuring too according to what one hears about its content I actually didn t read it Michael Crichton is better That Nietzsche was not included I m not sure about Schopenhauer has I believe the easy reason that there was no need of clarification that his works are non Catholic and so on the index by general rule without any need of explicity Of course theoretically those books Kant wrote on religion and that includes the Critic of pure r would be implicitly on the index as well because he was not a Catholic but from the very fact that he did believe in God the Congregation may have felt the status of his books from the Catholic point of view needed further clarification Also not every work of Kant was listed As to its importance as of now I agree as a layman and non theologian that reading some of the books today not only does not lead to ecclesiastic penalties as should be sure but also is not sinful in itself as no longer any disobedience to Church law likewise of course as to Card Bertone s semi censuring but it may be dangerous and thus imprudent which can be not free from sin 84 154 109 57