Reddit reviews The Almost Nearly Perfect People: Behind the Myth of the Scandinavian Utopia

In the book The Almost Nearly Perfect People Michael Booth gave the nationalism reason as why Sweden became less religious (and also socialist). He said (IIRC) that there was basically a movement that just rewrote what it meant to be Swedish and this included changing the religion. He doesn't support this with studies or anything but his explanation matches up very well with the nationalism explanation.