He was of the opinion that it was not possible to come to terms with this plan, not possible to ‘Catholicize’ it. Hence the genesis of the Encyclical Letter Quanta cura and the Sillabo, which are not to be seen separately from the profound theological meaning of the proclamation of the dogma of the Immaculate Conception, but, together with Vatican Council I, looked upon as Pious IX’s response to modern sin.
Pío IX creía que con este proyecto no se podía pactar, que no podía ser "catolicizado". He aquí la génesis de la encíclica Quanta cura y del Syllabus, que no hay que separar del profundo significado teológico de la proclamación del dogma de la Inmaculada Concepción, sino que hay que mirarlos, junto al Vaticano I, como la respuesta de Pío IX al pecado moderno.