Our Apostolic Mandate requires from Us that We watch over the purity of the Faith and the integrity of Catholic discipline. It requires from Us that We protect the faithful from evil and error; especially so when evil and error are presented in dynamic language which, concealing vague notions and ambiguous expressions with emotional and high-sounding words, is likely to set ablaze the hearts of men in pursuit of ideals which, whilst attractive, are nonetheless nefarious.
Pope Pius XIs it permitted for Catholics to be present at, or to take part in, conventions, gatherings, meetings, or societies of non-Catholics which aim to associate together under a single agreement everyone who, in any way, lays claim to the name of Christian? In the negative! ... It is clear, therefore, why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics. There is one way in which the unity of Christians may be fostered, and that is by furthering the return to the one true Church of Christ for those who are separated from Her.
Pope Pius XHoly Communion is the shortest and safest way to Heaven. There are others: innocence, but that is for little children; penance, but we are afraid of it; generous endurance of trials of life, but when they come we weep and ask to be. The surest, easiest, shortest way is the Eucharist.
Pope Pius X