Sách nói: Pascendi Dominici Gregis (On the Errors of the Modernists)

During a relatively short pontificate (1903 - 1914) Pope Pius X devoted much of his time to dealing with issues related to the rapid spread of secularism in the early Twentieth century. Pius eschewed the liberal approach of his immediate predecessor, Leo XIII, favouring instead the strict doctrinal guidelines established by Pope Pius IX in the Syllabus of Errors (1864). Pius X defined the root cause of the decline of the influence of the Catholic Church to be Modernism, a philosophical and artistic movement which came to prominence in the later Nineteenth Century.

The largest and most notable Encyclical of Pius X is Pascendi Dominici Gregis (On the Errors of the Modernists), published in 1907. This letter contains the famous statement, "Modernism is the synthesis of all heresies," and includes a lengthy appendix which contains a history of modernistic thought and refutations of the views held by its most celebrated representatives.