Singulari Quadam §2

² Accordingly, We first of all declare that all Catholics have a sacred and inviolable duty, both in private and public life, to obey and firmly adhere to and fearlessly profess the principles of Christian truth enunciated by the teaching office of the Catholic Church. In particular We mean those principles which Our Predecessor has most wisely laid down in the encyclical letter Rerum Novarum . We know that the Bishops of Prussia followed these most faithfully in their deliberations at the Fulda Congress of 1900. You yourselves have summarized the fundamental ideas of these principles in your communications regarding this question.