This most up-to-date document consists on a list of thesis condemned by Pope Pius IX. It condemns many errors broadly adopted in our times, such as pantheism, religious indifferentism (ecumenism), socialism, communism, laicism, etc.

Modernism is a heresy which has deeply penetrated into all segments of society, including – and mainly – ‘the very veins and heart of the Church’. The modernism was the last heresy condemned by the Church and the only condemnation in the whole XX^th century. In spite of that, its errors are found in the mentality of many clergymen of nowadays. In this encyclical, S. Pius X makes a masterly exposition of the modernist system, which can be defined as ‘the synthesis of all heresies’. It condemns, among others, agnosticism, religious immanence and its religious sentiment, evolution of the dogmas, etc.