Secondary sources, but check out Steve Bruce's work, especially God is Dead: Secularization in the West that's the first place I'd go for secularization. I might also check the first chapter and the early parts of the European chapters in Jose Casanova's Public Religion in the Modern World or whatever it's called. And for good measure, see what they say about religion in Thompson's the Making of the English Working Class.

That said, with references to primary sources, I don't know what primary sources you'd even expect to find if you're making an argument about all of Europe? I mean besides changing laws.