Question about European social & religious history

Dear Euro-friends (or other knowledgable persons):

One of the most significant differences between Europe & the US right now is that the USans are extremely religious by comparison. This is particularly striking because most USans have ancestors from Europe, speak a European language, and have a substantially European ("Western") culture -- we think of pre-1600 European history as "our" history, for instance.

What is your understanding of the time frame in which European religious practice faded away? What historical events were most important? Do you think European religiousity was similar to USan levels before WWI? WWII? Or did the change start further back?

I am currently reading James Sheehan's Where Have All the Soldiers Gone?: The Transformation of Modern Europe. He does *not* mention religion (or its lack) as being a factor in the post-WWII transforation of Europe into a civil state, defined by non-military institutions; do you think religion was involved? What do you (or the older people in your family) think about what caused the change?

References to sources available in English are especially appreciated, of course.