17 There are some shepherds, as you know, Venerable Brethren, who strive to lead away the sheep from this one fold and haven of salvation; you likewise know that this danger is daily growing greater. When We consider before God the immense number of men without the truth of the Gospel, and duly reckon the grave danger that faces many from the prevalence of atheistic materialism or from a certain so-called Christian creed which is infected by the tenets and errors of communism, We feel the deepest concern and solicitude that nothing be left undone to promote the work of the apostolate throughout the world. We make Our own the exhortation of the Prophet saying: "Cry, cease not, lift up thy voice like a trumpet."
Source: Evangelii Praecones (Vatican.va)