It is the duty of the Bishops to prevent writings of Modernists, or whatever savors of Modernism or promotes it, from being read when they have been published, and to hinder their publication when they have not. No books or papers or periodicals whatever of this kind are to be permitted to seminarists or university students. The injury to them would be not less than that which is caused by immoral reading-- nay, it would be greater, for such writings poison Christian life at its very fount. The same decision is to be taken concerning the writings of some Catholics, who, though not evilly disposed themselves, are ill-instructed in theological studies and imbued with modern philosophy, and strive to make this harmonize with the faith, and, as they say, to turn it to the profit of the faith. The name and reputation of these authors cause them to read without suspicion, and they are, therefore, all the more dangerous in gradually preparing the way for Modernism.
We order that you do everything in your power to drive out of your dioceses, even by solemn interdict, any pernicious books that may be in circulation there. The Holy See neglects no means to remove writings of this kind, but their number has now grown to such an extent that it is hardly possible to subject them all to censure. Hence it happens sometimes that the remedy arrives too late, for the disease has taken root during the delay... Nor are you to be deterred by the fact that a book has obtained elsewhere the permission which is commonly called the Imprimatur, both because this may be merely simulated, and because it may have been granted through carelessness or too much indulgence or excessive trust placed in the author, which last has perhaps sometimes happened in the religious orders. Besides, just as the same food does not agree with everyone, it may happen that a book, harmless in one place, may, on account of the different circumstances, be hurtful in another. Should a Bishop, therefore, after having taken the advice of prudent persons, deem it right to condemn any of such books in his diocese, We give him ample faculty for the purpose and We lay upon him the obligation of doing so. Let all this be done in a fitting manner, and in certain cases it will suffice to restrict the prohibition to the clergy; but in all cases it will be obligatory on Catholic booksellers not to put on sale books condemned by the Bishop. And while We are treating of this subject, We wish the Bishops to see to it that booksellers do not, through desire for gain, engage in evil trade. It is certain that in the catalogs of some of them the books of the Modernists are not infrequently announced with no small praise. If they refuse obedience, let the Bishops, after due admonition, have no hesitation in depriving them of the title of Catholic booksellers. This applies, and with still more reason, to those who have the title of Episcopal booksellers. (Papal Writings, Pascendi Dominici Gregis: On the Doctrine of the Modernists, 8 September 1907).