http://www.vatican.va/holy_father/paul_vi/encyclicals/documents/hf_p-vi_enc_06081964_ecclesiam_en.html
It is worth the time to read it in its entirety, but I would especially recommend paragraphs 99 and 100 where it speaks to atheism as a growing evil. We are still facing many of the same challenges today.
On this Feast of the Transfiguration of Our Lord Jesus Christ may we each be blessed abundantly through the overflowing grace of Almighty God!