The Easy Way
Religion is a very private thing in the Nordic countries and not something you're supposed to tell others about, both because most people aren't religious so it would be considered rude, and also because they're such personal feelings for religious people.
So a sure way to ruin the mood is to wear a T-shirt with a religious message, or any other object that somehow show what religion you belong to, most of all because Nordics will probably be too polite to talk about it (like I said, you're not supposed to talk about religion), so it will be a huge elephant in the room and they will just sit and stare awkwardly at you, afraid they might say something that'll hurt your religious feelings.
Sweden Iceland Norway America Denmark Finland
24th March 2012